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.





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