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CHAPTER ELEVEN
Nervous System II
C2
C3
C4
C5
C6
T1
C6
C7
S2
S3
C8
L1
L2
L3
L4
L5
T12
T1
S1
(a)
(b)
S5
C0
S4
S3
S2
S1
L5
L4
L3
L2
L1
L5
L1
C8
T1
T12
C7
C6
C5
C4
C3
C2
FIGURE 11.30
Dermatomes
(
a
) on the anterior body surface
and (
b
) on the posterior surface.
Spinal nerve C1 does not supply
any skin area.
pairs of
lumbar nerves
(numbered L1 to L5), F
ve pairs of
sacral nerves
(numbered S1 to S5), and one pair of
coccygeal
nerves
(Co).
The nerves arising from the superior part of the spinal
cord pass outward almost horizontally, whereas those from
the inferior portions of the spinal cord descend at sharp angles.
This anatomical organization is a consequence of growth. In
early life, the spinal cord extends the entire length of the ver-
tebral column, but with age, the column grows more rapidly
than the cord. Thus, the adult spinal cord ends at the level
between the F rst and second lumbar vertebrae, so the lumbar,
sacral, and coccygeal nerves descend to their exits beyond the
end of the cord. These descending nerves form a structure
called the
cauda equina
(horse’s tail) (F g. 11.29).
Each spinal nerve emerges from the cord by two short
branches, or roots, which lie within the vertebral column.
The
dorsal root
(posterior, or sensory, root) can be identi-
F
ed by an enlargement called the
dorsal root ganglion.
This
ganglion contains the cell bodies of the sensory neurons
whose axons (peripheral process) conduct impulses inward
from the peripheral body parts. The axons of these neurons
extend through the dorsal root and into the spinal cord (cen-
tral process), where they form synapses with dendrites of
other neurons (see F
g. 10.7).
An area of skin that the sensory nerve F
bers of a particu-
lar spinal nerve innervate is called a
dermatome.
Dermatomes
are highly organized, but they vary considerably in size and
shape, as
f
gure 11.30
indicates. A map of the dermatomes is
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