Nerve cells are so specialized that they have lost
the ability to repro- duce themselves. When a neuron dies, it cannot be
replaced. However, when an axon is damaged, if the damage is at some distance
from the cell body, the neuron will not wither away. The damaged section of theaxon may perish but often the remaining section will grow.
The growth may be
slow, although studies in some animals show a growth rate as fast as an inch a
week. Even when a neuron is completely destroyed, all is not necessarily lost;
sometimes other pathways may' come into use to serve the function of the
destroyed unit. " Man's endowment of neurons is vast. For example, the
ant, often considered to be a remarkably wise little insect, capable of
constructing cunning houses and leading a relatively complex social life, has
some 250 neurons.
Man has billions, and the number can be looked upon as The
Brain and Nervous System I 243 an indication of his tremendous potential if he
chooses to use them all, and wisely. Actually, man has two nervous systems, not
just one.
The Central Nervous System The central nervous system
includes the brain and spinal cord. The spinal cord is suspended in a cylinder
formed by the bones of the spine. The meninges, which cover the brain, also
continue down to serve as protective coverings for the cord. And cerebrospinal
fluid, between mid- dle and inner membranes, cushions the cord, just as it does
the brain, from shock.
The central nervous system connects to every part of the
body by means of 43 pairs of nerves. Twelve of these, the cranial nerves, go to
the eyes and other sense organs, the heart, and other internal organs. They are
called olfactory, optic, oculomotor, trochlear, trigeminal, abducens, facial,
auditory, glossopharyngeal, vagus, accessory, and hypoglossal.
The 31 other nervepairs go to skeletal muscles throughout the body. They branch off the cord between the bones of the spine. One of a pair goes to the right side of the body, the other to the left. Outside the cord, each nerve splits. One branch carries sensory-type fibers-for incoming messages-to the cord; the other carries motor-type fibers.
