Central spinal canal (or vertebral foramen) . (a) Sagittal section (b) Superior view. 1. Dorsal longitudinal ligament; 2. Spinolaminar line; 3. Yellow ligament; 4. Intervertebral foramen; 5. Pedicle; 6. Posterior wall; TD transverse diameter; FMSD fixed medial sagittal diameter, (c) expansion of the posterior wall by fracture or tumor
Peripherals (in the Horizontal Plane)
These are osteofibrous :
Anteriorly the canal is limited by the posterior aspect of the vertebral bodies, of the discs and the posterior longitudinal ligament (PLL). It is the posterior wall that can recoil under the effect of an axial compression (vertebral fracture) or of a vertebral tumor and compress the neural structures
Posteriorly by the laminae and the yellow ligaments (ligamentum flavum). These structures are stacked like roof tiles at the thoracic level but allow a gap between them at the cervical and lumbar level thus providing access for needle puncture under the occiput or lumbar level
Laterally by the pedicles separated by intervertebral foramina.
The spinal canal follows the spinal curvatures, but its diameter varies widely depending on the segment. There are two diameters:
anteroposterior median (or fixed medial sagittal diameter, FMSD) between the posterior wall and the union of the laminae and the spinous process (spinolaminar line)
transverse between the pedicles.
In reality, it is more interesting to determine the surface of the canal.
The latter is wide at the cervical and lumbar level and less at the thoracic level, especially at T9. Dimeglio  (Fig. 2) reminds us that the high cervical vertebra can fit the thumb, whereas the lumbar vertebra can fit the index and the thoracic vertebra the little finger.
We admit that the minimum anteroposterior diameter is:
Average 20 mm upper cervical, 12 mm lower cervical,
Average 10 mm thoracic,
Average 15 mm in lumbar.
The Reserve Volume (RV)
In reality, more than the anteroposterior or transverse diameter, the reserve volume (RV) around the dura mater is most important: it is defined in the lumbar region by Lee  but can be used at all levels (Fig. 3). This RV is relatively important in the cervical and lumbar spine and is lower in the thoracic area despite the small diameter of the thoracic spinal cord (Fig. 4). The most classic example is that of the C1C2 region where the spinal cord occupies one-third of the canal, the odontoid process (peg) another third, the last third corresponding to free space for the cord, the RV (rule of thirds): spinal trauma lesions, having survived the trauma, are rare at this level sometimes despite large traumatic osteoarticular movements (Fig. 5).
The Cervical Spinal Canal
It is the widest; in fact, its median sagittal diameter decreases from top to bottom since it is on average 22 mm in relation to C1, 20 mm in relation to C2, and 14–17 mm in the lower cervical level (Fig. 6).
The fixed medial sagittal diameter (FMSD) corresponds to the solid arrows in Fig. 6 drawn between the middle of the posterior vertebral body and the upper part of the lamina: this bone diameter is fixed and said to be “constitutional” since in principle its size does not vary after the closure of the neurocentral cartilage, around 5–6 years. The dotted arrow joining the yellow ligament at disc C6C7 is called the mobile medial sagittal diameter (MMSD) since it corresponds to the Junghanns mobile segment; in degenerative cervical (or lumbar) pathology, this segment is the most affected. Classically, a cervical spinal canal of normal size is the sagittal diameter of a cervical vertebral body (Wackenheim ) (Fig. 7).
Specifically, Pavlov et al.  provide an index which is the ratio of A (FMSD) and B (anteroposterior diameter of vertebral body). If this index is less than 0.8, there is canal narrowing (Fig. 8).
Finally, on a lateral radiograph , we can distinguish 3 portions of 5 in the lateral projection of the cervical canal which is located between the body (portion 1) and the spinous process (portion 5) (Fig. 9).
portion 2 is the pedicle (transverse),
portion 3 is articular,
portion 4 is lamina; it is often considered a safe space.
Thoracic Spinal Canal
It is oval as at the cervical level but much smaller (FMSD 10 to 12 mm).
Lumbar Spinal Canal
It also represents a FMSD drawn between the middle of the posterior vertebral wall (more precisely the posterior longitudinal ligament) and the cranial part of the spinous process which is more ventral than the caudal part; on this caudal part is inserted the yellow ligament which goes up high on the cranial lamina in the interlaminar space.
The MMSD is traced between the posterior edge of the annulus and the yellow ligament: thus a bulging disc, hypertrophy of the yellow ligament or the thickened joint capsule frequently reduces the MMSD in lumbar degenerative pathology (Fig. 10).
For Verbiest , there is absolute stenosis if the measurement (which at the time was performed intraoperatively) is less than 10 mm and relative stenosis if this measurement is between 10 and 12 mm. If the measurement is radiological, the canal is said to be wide if the FMSD is greater than 14 mm and narrow if it is less than 12 mm. Ullrich  sets the limit at 11.5 mm for the anteroposterior diameter, 16 mm for the interpedicular diameter, and 145 mm2 for the surface area.
The shape of the lumbar spinal canal changes from L1 to L5; it is rather predominantly ovoid at its upper part (as at the cervical and thoracic levels). Due to the imprint of the articular processes which are becoming increasingly coronal, there appears a side portion of the spinal canal which is called lateral recess as described later. The lumbar canal thus becomes triangular (or deltoid, or trefoil), if the impression of the articular is more marked  (Fig. 11). It is as if the lumbar spinal canal tends to extend transversely and to shrink from front to back.
Beside the spinal canal, we can describe:
the lateral recess which are a lateral expansion of the spinal canal,
the intervertebral foramina, lateral openings towards the outside of the spinal canal,
the transverse foramina that exists only at the cervical level and contains the vertebral artery.
The Lateral Recess
It is defined as a lateral expansion of the spinal canal limited laterally by the pedicle and posteriorly by the articular facets; it is not entirely integral at the cervical and thoracic levels where the spinal canal is oval. It becomes increasingly marked as one descends the lumbar spine towards the lumbosacral junction where the impression of the S1 articular facet, oriented coronally, is marked in the lumbar spinal canal (Fig. 12).
Considering the osseous morphology, the lateral recess is therefore limited laterally by the pedicle, anteriorly by the vertebral body directly within the pedicle, posteriorly by the superior articular facet, and inferiorly by the isthmus. We can thus oppose the lateral recess superior to the superior articular (pediculoarticular portion in tomodensimetry) and the lateral recess to the isthmus (pediculolamary portion) (Fig. 13).