e. A very narrow canal, called the central canal, is located in the center of the
spinal cord. This central canal is continuous with the fourth ventricle of the brain and
f. The processes of the neurons that compose the surrounding white matter are
grouped into pathways called fiber tracts.
Tracts conducting impulses from the brain are called motor tracts.
Tracts conducting impulses to the brain are called sensory tracts.
(3) At some specific point along the neuraxis, these pathways cross to the
opposite side of the cord and continue their path. (Each crossing is called a
decussation.) Thus, the right cerebral hemisphere of the brain communicates with the
left half of the body, and the left cerebral hemisphere communicates with the right half
of the body.
g. Thirty-one pairs of spinal nerves exit from the spinal cord.
THE PERIPHERAL NERVOUS SYSTEM
a. Connecting the CNS to all parts of the body are nerves. A nerve is a
collection of neuron processes, grouped together, and located outside of the CNS.
(Neuron processes, grouped together, and inside the CNS are the fiber tracts of the
spinal cord.) Nerves outside the CNS are referred to as peripheral nerves, or the PNS.
These nerves connect the CNS to the periphery of the body.
b. The PNS is made up of a large number of nerves arranged in pairs. Each
pair includes one nerve for the left side of the body and one nerve for the right side.
(1) Peripheral nerves connected to the brainstem are called cranial nerves.
They are numbered from I through XII and have individual names.
(2) Peripheral nerves connected to the spine are called spinal nerves. They
are identified by a letter, representing the corresponding region of the vertebral column,
and a number representing the sequence within the region. For example, L-5 is the fifth
spinal nerve in the lumbar region.
THE CRANIAL NERVES
a. Olfactory Nerve (I).
Transmits smell impulses from receptors in the nasal mucosa to the