Section III. THE PERIPHERAL NERVOUS SYSTEM (PNS)
Connecting the CNS to all parts of the body are individual organs known as
nerves. A nerve is a collection of neuron processes together and outside of the CNS.
Peripheral nerves are nerves which pass from the CNS to the periphery of the body.
Together, they are referred to as the peripheral nervous system.
a. These nerves are bilateral and segmental.
(1) Bilateral. This means that the peripheral nerves occur in pairs. In each
pair, there is one nerve to the right and one to the left.
(2) Segmental. The pairs of peripheral nerves occur in intervals,
corresponding to the segments of the human embryo.
b. Peripheral nerves connected to the brainstem are called cranial nerves.
They are numbered from I through XII and also have individual names.
c. Peripheral nerves connected to the spinal cord are called spinal nerves.
They are identified by a letter representing the region of the vertebral column and a
number representing the sequence in the region:
(1) Cervical: C-1 through C-8.
(2) Thoracic: T-1 through T-12.
(3) Lumbar: L-1 through L-5.
(4) Sacral: S-1 through S-5.
Thus, there are 31 pairs of spinal nerves.
A "TYPICAL" SPINAL NERVE (FIGURE 12-7)
In the human body, every spinal nerve has essentially the same construction
and components. By learning the anatomy of one spinal nerve, you can understand the
anatomy of all spinal nerves. Like a tree, a typical spinal nerve has roots, a trunk, and