In the matter of length, great variation is found among the axons in
different parts of the body. In certain parts of the brain, for example,
are fibers not more than one one-hundredth of an inch in length, while the
axons that pass all the way from the spinal cord to the toes have a length
of more than three feet. Between these extremes practically all variations
in length are found.
*Arrangements of the Neurons.*--Nowhere in the body do the neurons exist
singly, but they are everywhere connected with each other to form the
different structures observed in the nerve skeleton. Two general plans of
connection are to be observed, known as the anatomical and the
physiological, or, more simply speaking, as the "side-by-side" and
"end-to-end" plans. The side-by-side plan is seen in that disposition of
the neurons which enables them to form the nerves and the ganglia, as well
as the brain and spinal cord. The end-to-end connections are necessary to
the work which the neurons do.
*Side-by-side Connections.*--On separating the ganglia and nerves into
their finest divisions, it is found that the nerves consist of axons,
while the ganglia are made up mainly of cell-bodies and dendrites. The
axons lie side by side in the nerve, being surrounded by the same
protective coverings, while the cell-bodies form a rounded mass or
cluster, which is the ganglion (Fig.
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