FIG. 7.—Longitudinal (A) and transverse (B) section of nerve fiber. The heavy border
represents the medullary, or enveloping sheath, which becomes thicker in the larger fibers.—
After DONALDSON.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.