the peripheral ones. One may illustrate this important relation by
a telegraph system. The message a railroad operator sends out--_e.g._,
that which determines whether a train is to be held at a certain
station or sent on--might depend wholly on information received from
another office. The extra flow of blood to the stomach when food
enters it is owing to such a relation of things. The food acts as a
stimulus to the ends of the nerve-fibres, and, in consequence, there
is an ingoing (_afferent_) message or impulse, and, by reason of this,
an outgoing (_efferent_) one to the muscle-cells of the small
blood-vessels, owing to which they contract less strongly and the
calibre of these vessels is increased; hence more blood reaches the
smallest vessels of all (_capillaries_.) Such a physiological relation
of things is termed _reflex action_. For such reflex action there are
required structurally at least two neurones or nerve-cells, and
functionally a stimulus of a certain strength and quality. Of course,
if more blood passes to the stomach there must be less somewhere else,
as the total volume of the blood is limited. The value of the
knowledge of such a fact is obvious. It must be unwise to exercise
vigorously immediately after meals, for this determines blood to the
muscles which would serve a better purpose in the digestive organs.
For a like reason the singer who would do his best before the public
will refrain from taking a large meal before appearing.

As this subject of reflex action is of the highest importance, the
reader is advised to make himself thoroughly familiar with the
principles involved before perusing the future chapters of this work.
Fig. 16 shows the structural relations for reflex action. It also
indicates how such nervous relations may be complicated by other
connections of the nerve-cells involved in the reflex action. It will
be seen that they make many upward connections with the brain, in
consequence of which consciousness may be involved. Ordinarily one is
more or less conscious of reflex action, though the will is not
involved; in fact, a willed or voluntary action is usually considered
the reverse of a reflex or involuntary action. But for a reflex action
the brain is not essential. As is well known, a snake's hinder part
will move in response to a touch when completely severed from the head
end; and movements of considerable complexity can be evoked in a
headless frog.

Herein, then, lies t