The Parts of a Typical Vertebra and Their Functions

The vertebral column is made up of 33 vertebrae, arranged as follows: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal. The sacral and the coccygeal vertebrae unite to form composite bones, called the os sacrum and os coccygis. The 5 sacral vertebrae have completely fused to form a single mass by the 23rd year; a gap, however, often persists between the 1st and 2nd sacral bodies until the 32nd year (McKern and Stewart). The last three pieces of the coccyx fuse together in middle life and these in turn fuse with the first piece still later. There are, therefore, 24 presacral or true vertebrae, of which 12 bear ribs and 12 do not.

Not only have the bones of each region features characteristic of their particular region but every bone in each region features characteristic of their particular region but every bone in each region has one or more distinguishing features of its own.

A vertebra is composed of the following parts (fig. 16):

• A weight-bearing part - the body.
• A part that protects the spinal cord - the vertebral arch.
• Three levers on which muscles pull - the spinous process; the right and the left transverse process.
• Four projections which restrict movements - two superior and two inferior articular processes.

Fig 16: The parts of a vertebra, from above

Fig 17: A typical vertebra (side view)

Body

The body of a vertebra is functionally and structualy comparable with a long bone (fig. 17). Thus, it resembles the long bones of the limbs in that it is weight supporting, constricted about its middle, enlarged at its two ends, and in that its two ends are articular; also in having a primary center of ossification for the “diaphysis” which appears early, and secondary centers for the upper and lower epiphyses, which however appear unduly late for pressure epiphyses. It is, indeed, a long bone in miniature.

Its early development is peculiar: during the precartilaginous state, each of a series of cubical somites that develop round the notocord splits into a cranial and a caudal half; thereupon the adjacent halves of two somites unite to form a body. Large vascular foramina persist behind and at the sides of adult vertebral bodies and indicate the site of union of two half somites.

Vertebral Arch

The vertebral arch protects the spinal medulla (spinal cord) from injury, as the roof of the cranium protects the brain. Immediately behind its attachment to its body, each half of the arch is crossed, both above and below, by a spinal nerve. Accordingly, this part of the arch, called the pedicle (root) is grooved above and below, but especially below, to allow ample space for the passage of a nerve. The grooves are the superior and inferior vertebral noteches. The posterior band-like portions of each arch, the right and left laminae, meet behind in the median plane. A vertebralarch and the posterior aspect of a body enclose a space, the vertebral foramen, in which the spinal medulla (spinal cord) and its membranes are lodged.

Transverse and Spinous Processes

The movement of one body on another is effected in part through the actions of muscles on the lever-like transverse and spinous processes, which project like the spokes of a capstan. The transverse processes project laterally on each side from the junction of a pedicle and a lamina; the spinous process or spine projects backwards in the median plane from the site of union of a right and left lamina.

Articular Processes

The, so-called, articular processes arise near the junction of pedicle and lamina. The superior processes spring rather from the pedicles and face in a backward direction (backwards and upwards in the cervical region; baclwards and laterally in the thoracic region; backwards and medially in the lumbar region) whereas the inferior articular processes spring from the laminae and face in the contrary direction. It is evident that in all regions the contact established between upper and lower articular processes prevents forward displacement of an upper vertebra on a lower.

The upper and lower surfaces of the bodies are the real articular surfaces of the vertebrae. The articular processes (except in the case of the atlas and axis) do not transmit weight. Their presence interferes with the unrestricted mobility the bodies might otherwise enjoy and decrees in what direction movements between two adjacent vertebrae shall be allowed. THey play the part which in the joints of the limbs is assigned to ligaments, namely that of restricting movement. There are, however, circumstances in which they bear weight, e.g. on rising from the stooping position. L. 5 is “normally” in a stooping position.

Collectively the vertebral foramina constitute the vertebral canal. Collectively two adjacent vertebral notches constitute an intervertebral foramen. Entering into the composition of an intervertebral foramen are: above and below, pedicles; in front, an intervertebral disc and parts of the two bodies it unites; and behind, two articular processes and the capsule uniting them (fig. 18).

Fig 18: Composition of an intervertebral foramen

Ossification

At birth a vertebra is in three parts - a centrum and the right and left sides of a neural arch, united to each other by hyaline cartilage (fig. 19). The site of union of a centrum and a neural arch is a neuro-central synchondrosis.

Fig 19: A vertebra at birth

The facets for the heads of the ribs are situated on the neural arches just behind the neuro-central synchondroses - they are not on the sides of the centra.

Synostosis of the two halves of the arch takes place posteriorly during the 1st year, and of the arch and centrum between the 3rd and 6th years.

Epiphyses

Pressure and traction epiphyses appear about puberty and fuse not later than the 24th year. In most mammals the pressure epiphyses take the form of plates, but in man they are rings, that overlie the upper and lower surfaces of the centrum and extend on to the neural arch (fig. 22).

The body of a vertebra is a composite of the upper and lower epiphyses and the mass of bone between them.

It includes the centrum, parts of the neural arch, and the facets for the heads of the ribs. The terms “body” and “centrum” are not, therefore, strictly speaking interchangeable; neither are the terms “vertral arch” and “neural arch”.

Scale-like traction epiphyses appear on the tips of the spinous, transverse and (in the lumbar region) mamillary processes.