130 Neuroanatomy: Draw It to Know It
The Spinal Canal
Here, we will draw a sagittal view of the spinal canal,
which encases the spinal cord and spinal nerve roots. To
begin, draw a sagittal section through the brainstem and
then the spinal cord. Indicate that the cervical segment
of the spinal cord angles anteriorly; the thoracic segment
angles posteriorly; and the lumbosacral and coccygeal
segments angle anteriorly, again.
Th e vertebral column contains 33 vertebrae: 7 cervi-
cal, 12 thoracic, and 5 lumbar vertebrae, and then the
vertebral fusions that constitute the sacrum and coccyx.
Th e spinal cord ends at the L1–L2 vertebral column
level; thus the spinal cord is shorter than the spinal canal,
and therefore, at any vertical height, the spinal cord level
is lower than the surrounding vertebral level. Now, show
the following vertebral bodies: C1, C2, C7, T1, T12,
and L1; then, show that there is hyperfl exion of the sacral
column at the clinically important L5–S1 junction, a
common site of nerve root compression; then, show that
S5 angles further posteriorly; and fi nally show that the
coccyx angles back anteriorly.^1
A pair of motor and sensory roots exit the spinal cord
at each level and combine within an intervertebral neural
foramen to form a spinal nerve. Th ere are a total of 31
spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral,
and 1 coccygeal. Show that the fi rst cervical spinal nerve,
C1, exits below the skull (beneath the foramen magnum)
and above the fi rst cervical vertebra. Th en, indicate that
C2 exits above its vertebra, and so on — show that C7
exits above the C7 vertebra, which is the lowermost cer-
vical vertebra. As mentioned, however, there are 8 cervi-
cal spinal nerves, so show that C8 exits underneath the
C7 vertebra and above the T1 vertebra. Where, then,
must the T1 nerve exit? Show that the T1 spinal nerve
exits below its corresponding vertebra because C8 fi lls
the space above it. Th en, show that T12 exits beneath
the T12 body, L1 beneath L1, L5 under L5, S1 under S1,
S5 under S5, and the coccygeal nerve exits underneath
the coccyx. Only C1 through C7 exit above their related
vertebral bodies; the rest exit beneath them.
Now, let’s label a few important caudal spinal canal
structures. Indicate that the collection of nerve fi bers
that traverses the caudal spinal canal is the cauda equina,
which is the Latin term “tail of the horse.” Th en, indicate
that the anatomically related, threadlike fi brous tissue
that extends from the distal tip of the spinal cord through
the caudal spinal canal is the fi lum terminale. Finally,
label the most distal, bulbous region of the spinal cord
as the conus medullaris. Th e cauda equina and conus
medullaris each have related syndromes, which are self-
named. Cauda equina syndrome is a lower motor neuron
syndrome, whereas conus medullaris syndrome is an
upper motor neuron syndrome. Injury to both the cauda
equina and conus medullaris causes mixed upper and
lower motor neuron disease.
Next, let’s label the meningeal coverings of the spinal
canal. First, label the surface of the spinal cord as the
pia mater. Th en, draw a combined layer of dura and
underlying arachnoid mater. Show that there is plenty of
separation between these meningeal coverings and the
underlying pia mater. Label the naturally occurring space
between the arachnoid and pia mater layers as the suba-
rachnoid space. Th en, show that the site of fl uid collec-
tion during a lumbar puncture is in the lumbar cistern,
the subarachnoid space below the level of the spinal cord.
Finally, draw the vertebral arch (the posterior portion of
the vertebral column), and between it and the dura
mater, label the epidural space. Note that although we
have only labeled the posterior aspect of the epidural
space, here, throughout the spinal canal, the spinal epi-
dural space exists superfi cial to the dura mater and inter-
nal to the periosteal lining of the vertebral canal. Th e
spinal epidural space is a common site for hematoma,
infection, and spread of neoplastic disease.^2 – 4