cal organs are subject to
involuntary contractions, the causes of these contractions must be
sought through an investigation of the subject of reflex actions.

Reflex actions are of several kinds; of these the simplest type, and the
one most easily studied, is the muscular contraction due to the
excitation of the sensory nerve endings located in the skin. Thus when
the sole of the foot of a sleeping person is tickled, the leg is at
first drawn up and then violently kicked out. An exhaustive discussion
of the physiological and psychological features of reflex action is not
called for here; a sufficient understanding of the subject may safely be
assumed to be possessed by the reader.

Involuntary muscular contractions often occur as reflex actions without
any direct or tactual irritation of the sensory nerve endings. Several
examples of this form of reflex action are now to be considered. These
actions will be seen to be matters of such common experience as to call
for no special proof. They are the following:

(_a_) Reflex actions performed under the influence of sensory
impressions other than those of touch or muscular sense.

(_b_) Involuntary muscular contractions due to nervousness.

(_c_) Contractions of the muscles of certain members, caused by the
turning of the attention specially to the members.

(_d_) Involuntary contractions of muscles, accompanying the exertion of
other associated and antagonist muscles, and due to the radiation of
nerve impulse.

(_a_) _Reflex Actions due to Sensory Impressions other than those of
Touch or Muscular Sense_

A wide range of movements is included under this heading. Of these it is
necessary to mention only a few, such as the sudden start on the hearing
of an unexpected noise, the instinctive movement of dodging to escape an
approaching missile, and the raising of the arm to ward off an expected
blow.

Actions of a somewhat similar character normally occur in which it is
not easy to point to the excitation of any sense or senses. These
include the instinctive cowering attitude of fear, the play of facial
expression caused by sentiment and emotion, etc.

(_b_) _Involuntary Actions due to Nervousness_

A condition of marked nervousness generally causes the involuntary
contraction of muscles. Who does not recall his earliest attempts at
"speaking a piece" in school? The trembling of the lips, the twitching
of the arms and hands, and the vain attempts to govern the