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Structural Elements Of The Nervous System








It will help in understanding both the structure and the working of the
nervous system to keep in mind that it contains but one fundamental
unit of structure. This is the neurone. Just as the house is built up
by adding brick upon brick, so brain, cord, nerves and organs of sense
are formed by the union of numberless neurones.


from a to e. In a, the elementary cell body alone is present; in
c, a dendrite is shown projecting upward and an axon downward.--After
DONALDSON.]

THE NEURONE.--What, then, is a neurone? What is its structure, its
function, how does it act? A neurone is a protoplasmic cell, with its
outgrowing fibers. The cell part of the neurone is of a variety of
shapes, triangular, pyramidal, cylindrical, and irregular. The cells
vary in size from 1/250 to 1/3500 of an inch in diameter. In general the
function of the cell is thought to be to generate the nervous energy
responsible for our consciousness--sensation, memory, reasoning, feeling
and all the rest, and for our movements. The cell also provides for the
nutrition of the fibers.

NEURONE FIBERS.--The neurone fibers are of two kinds, dendrites and
axons. The dendrites are comparatively large in diameter, branch
freely, like the branches of a tree, and extend but a relatively short
distance from the parent cell. Axons are slender, and branch but little,
and then approximately at right angles. They reach a much greater
distance from the cell body than the dendrites. Neurones vary greatly in
length. Some of those found in the spinal cord and brain are not more
than 1/12 of an inch long, while others which reach from the extremities
to the cord, measure several feet. Both dendrites and axons are of
diameter so small as to be invisible except under the microscope.

NEUROGLIA.--Out of this simple structural element, the neurone, the
entire nervous system is built. True, the neurones are held in place,
and perhaps insulated, by a kind of soft cement called neuroglia. But
this seems to possess no strictly nervous function. The number of the
microscopic neurones required to make up the mass of the brain, cord and
peripheral nervous system is far beyond our mental grasp. It is computed
that the brain and cord contain some 3,000 millions of them.

COMPLEXITY OF THE BRAIN.--Something of the complexity of the brain
structure can best be understood by an illustration. Professor Stratton
estimates that if we were to make a model of the human brain, using for
the neurone fibers wires so small as to be barely visible to the eye, in
order to find room for all the wires the model would need to be the size
of a city block on the base and correspondingly high. Imagine a
telephone system of this complexity operating from one switch-board!

GRAY AND WHITE MATTER.--The gray matter of the brain and cord is
made up of nerve cells and their dendrites, and the terminations of
axons, which enter from the adjoining white matter. A part of the mass
of gray matter also consists of the neuroglia which surrounds the nerve
cells and fibers, and a network of blood vessels. The white matter of
the central system consists chiefly of axons with their enveloping or
medullary, sheath and neuroglia. The white matter contains no nerve
cells or dendrites. The difference in color of the gray and the white
matter is caused chiefly by the fact that in the gray masses the
medullary sheath, which is white, is lacking, thus revealing the ashen
gray of the nerve threads. In the white masses the medullary sheath is
present.





Next: Gross Structure Of The Nervous System

Previous: The Mind's Dependence On The External World



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