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. 128). But the axons, in order to
connect with the cell-bodies, must terminate within the ganglion, so that
they too form a part of it. To some extent, also, axons pass through
ganglia with which they make no connection. The neurons in the brain and
spinal cord also lie side by side, but their arrangement is more complex
than that in the nerves and ganglia.

[Fig. 128]

Fig. 128--*Diagrams illustrating arrangement of neurons.* _A, B._ Ganglia
and short segments of nerves. 1. Ganglion. 2. Nerve. In the ganglion of
_A_ are end-to-end connections of different neurons; in the ganglion of
_B_ are the cell-bodies of di-axonic neurons. _C._ Section of a nerve
trunk. 1. Epineurium consisting chiefly of connective tissue. 2. Bundles
of nerve fibers. 3. Covering of fiber bundle, or perineurium. 4. Small
artery and vein.

The side-by-side arrangement of the neurons shows clearly the structure of
the ganglia and nerves. The nerve is seen to be a bundle of axons, or
nerve fibers, held together by connective tissue, while the ganglion is
little more than a cluster of cell-bodies. Their connection is necessarily
very close, for the same group of neurons will form, with their axons, the
nerve, and, with their cell-bodies, the ganglion (Fig. 128).

*End-to-end Connections.*--These consist of loose end-to-end unions of the
fiber branches of