in
Chapters XX, XXI, and XXII.

*How External Stimuli act on Internal Organs.*--For stimulating the neurons
not connected with the body surface we are dependent, so far as known,
upon the nervous impulses. An impulse started by the external stimulus
goes only so far as its neuron extends. But it serves as a stimulus for
the neuron with which the first connects and starts an impulse in this
connecting neuron, the point of stimulation being where the fiber
terminations of the first neuron make connection with the dendrites of the
second. This impulse in turn stimulates the next neuron, and so on,
producing a series of impulses along a given nerve path. In this way the
effect of an external stimulus may reach and bring about action in any
part of the body. This is in brief the general plan of inducing action in
the various organs of the body. This plan, however, is varied according to
circumstances, and at least three well-defined forms of action are easily
made out. These are known as _reflex action, voluntary action_, and
_secondary reflex action_.

*Reflex Action.*--When some sudden or strong stimulus acts upon the nerve
terminations at the surface of the body, an immediate response is
frequently observed in some quick movement. The jerking away of the hand
on accidentally touching a hot stove, the winking of the eyes on sudden
exposure to danger, and the quick movements from slight electrical shocks
are familiar examples. The explanation of reflex action is that external
stimuli start impulses in neurons terminating at the surface of the body
and these, in turn, excite impulses in neurons which pass from the spinal
cord or brain to the muscles (Fig. 138). Since there is an apparent
turning back of the impulses by the cord or brain, the resulting movements
are termed _reflex_.(105)

[Fig. 138]

Fig. 138--*Diagram illustrating reflex action of an external organ.*

*Reflex Action and the Mind.*--If one carefully studies the reflex actions
of his own body, he will find that they occur at the time, or even a
little before the time, that he realizes what has happened. If a feather
is brought in contact with the more sensitive parts of the face of a
sleeping person, there is a twitching of the skin and sometimes a movement
of the hand to remove the offending substance. Surgeons operating upon
patients completely under the influence of chloroform, and therefore
completely unconsciou