ities of our daily life.

Even walking, riding the bicycle, playing on a piano, and numberless other
such acts may be reflex movements. To learn how, requires, of course, the
action of the brain, but with frequent repetition the muscles become so
accustomed to certain successive movements, that they are continued by
the cord without the control of the brain. Thus we may acquire a sort of
artificial reflex action, which in time becomes in a way a part of our
organization, and is carried on without will power or even consciousness.

So, while the hands are busily doing one thing, the brain can be intently
thinking of another. In fact, any attempt to control reflex action is more
apt to hinder than to help. In coming rapidly down stairs, the descent
will be made with ease and safety if the spinal cord is allowed entire
charge of the act, but the chances of stumbling or of tripping are very
much increased if each step be taken as the result of the will power. The
reflex action of the cord may be diminished, or inhibited as it is called,
but this power is limited. Thus, we can by an effort of the will stop
breathing for a certain time, but beyond that the reflex mechanism
overcomes our will and we could not, if we would, commit suicide by
holding our breath. When we are asleep, if the palm of the hand be
tickled, it closes; when we are awake we can prevent it.

[Illustration: Fig. 120.--Dr. Waller's Diagrammatic Illustration of the
Reflex Process.

From the sentient surface (1) an afferent impulse passes along (2) to the
posterior root of the spinal cord, the nerve fibers of the posterior root
ending in minute filaments among the small cells of this part of the cord
(3). In some unknown way this impulse passes across the gray part of the
cord to the large cells of the anterior root (5), the cells of this part
being connected by their axis-cylinder with the efferent fibers (6). These
convey the stimulus to the fibers of the muscle (7), which accordingly
contract. Where the brain is concerned in the action the circuit is longer
through S and M.]

Experiment 131. _To illustrate reflex action by what is called
knee-jerk._ Sit on a chair, and cross the right leg over the left one.
With the tips of the fingers or the back of a book, strike the right
ligamentum patellae. The right leg will be raised and thrown forward with
a jerk, owing to the contraction of the quadriceps muscles. An
appreciable time elapses betwee