olds the whole body together in one
harmonious whole; it is the governing organization of the multicellular
community (Section 55), and the supreme head of the government
resides in the brain, and is called the mind. But just as in a political
state only the most important and most exceptional duties are
performed by the imperial body, and minor matters and questions of
routine are referred to boards and local authorities, so the mind
takes cognisance only of a few of the higher concerns of the animal,
and a large amount of the work of the nervous system goes on
insensibly, in a perfectly automatic way-- even much that occurs in
the brain.

Section 101. The primary elements in the tissue of the nervous
system are three; nerve fibres, which are simply conducting threads,
telegraph wires; ganglion cells, which are the officials of the system;
and neuroglia, a fine variety of connective tissue which holds these
other elements together, and may also possibly exercise a function in
affecting impressions. A message along a nerve to a ganglion cell is
an afferent impression, from a cell to a muscle or other external end
is an efferent impression. The passage of an impression may be
defined as a flash of kataboly along the nerve, and so every feeling,
thought, and determination involves the formation of a certain
quantity of katastases, and the necessity for air and nutrition.

Section 102. Unlike telegraph wires, to which they are often
compared, nervous fibres usually convey impressions only in one
direction, either centrally (afferent or sensory nerve fibres), or
outwardly (efferent or motor nerve fibres). But the so-called motor
nerve fibres include not only those that set muscles in motion, but
those that excite secretion, check impulsive movements, and govern
nutrition.

Section 103. Figure 7, Sheet 8, shows the typical structure of nervous
tissues. The nerve fibres there figured are bound together by
endoneurium into small ropes, the nerves, encased in perineurium.
There is always a grey axis cylinder (a.c.), which may (in medullated
nerves), or may not (in non-medullated or grey nerves) have a
medullary sheath (s.S.) interrupted at intervals by the nodes of
Ranvier (n.R.). Nuclei (n.) at intervals under the sheath indicate the
cells from which nerve fibres are derived by a process of elongation.
The nerves of invertebrata, where they possess nerves, are mostly
grey, and so are those of the sympathetic system o