*The Peripheral Division.*--The peripheral division of the nervous system
includes all the nervous structures found outside of the brain and spinal
cord. These consist of the cranial, spinal, and sympathetic nerves, and of
various small ganglia, all of which are closely connected with the central
system.
*Spinal Nerves and Dorsal-root Ganglia.*--The spinal nerves comprise a
group of thirty-one pairs, which connect the spinal cord with different
parts of the trunk, with the upper, and with the lower extremities. Each
nerve joins the cord by two roots, these being named from their positions
the _ventral_, or anterior, root and the _dorsal_, or posterior, root. The
two roots blend together within the spinal cavity to form a single nerve
trunk, which passes out between the vertebrae. On the dorsal root of each
spinal nerve is a small ganglion which is named, from its position, the
_dorsal-root ganglion_. (Consult Figs. 133 and 135, and also Fig. 125.)
*Double Nature of Spinal Nerves.*--Charles Bell, in 1811, made the
remarkable discovery that each spinal nerve is double in function. He
found the portion connecting with the cord by the dorsal root to be
concerned in the _production of feeling_ and the portion connecting by the
ventral root to be concerned in the _production of motion_.
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