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72 The spinal cord

C2
C3
C4
C2
C5
C2 C6
C3 C3 C7
C8
C4 T3 C4 T1
T2 T3 T4 T2
T4 T5
T6
T5 T7
T6 T8 T2
T7 T9
T10
T8 T11 C5
C5 T1 T9 T12 L1
T10 L1 L2
T1
T11 L3
C6 T12 S1 C6 L4
C7 L1 S3 L2 C7 L5
S1
S4 S2
C8 S5 C8 S3
S4
L2 S5

L3 L3 Fig.72.1
L4 S2 A map of the dermatomes.
The small diagram shows the regular arrangement
of dermatomes in the embryo, before the limbs
become fully developed
L5 L4 L5

S1
S1

L5

In the fetus, the spine and the spinal cord are the same length but, since The spinal cord shows two enlargements in the cervical and lumbar
the spine grows more quickly than the spinal cord, the lower end of the regions, corresponding to the origins of the nerves that make up the
cord gradually retreats upwards, reaching the level of L3 at birth and limb plexuses. It is for this reason that the vertebral canal is larger in
the lower border of L1 in the adult. For this reason, too, the anterior and these regions, and they are also the regions of the greatest mobility.
posterior nerve roots become more and more oblique from above down- • The meninges: the meningesathe dura, pia and arachnoidaare con-
wards so the lumbar and sacral nerve roots form a bundle, the cauda tinuous with those of the cranial cavity so that the subarachnoid space
equina, which occupies the lower part of the spinal canal. The posterior is also continuous and cerebrospinal fluid may be drained from the sys-
root ganglia occupy the intervertebral foramina so that the roots do tem or its pressure measured, by lumbar puncture. The subarachnoid
not unite until after this point. The spinal cord itself ends as the filum space ends at the level of S2.
terminale, a thin fibrous band which is included in the cauda equina.
160 Spine and spinal cord