veral muscles are
made to work together harmoniously, and also how it is possible that a
pin prick, directly affecting just a few sensory axons, causes a big
movement of many muscles. Well, we find the sensory axon, as it enters
the cord, sending off a number of side branches, each of which
terminates in an end-brush in synaptic connection with the dendrites
of a motor nerve cell.

[Illustration: Fig. 10.--Cooerdination brought about by the branching
of a sensory axon. (Figure text: cord, sensory neurone, motor neurone)]

Thus the nerve current from a single sensory neurone is distributed to
quite a number of motor neurones. Where there are central neurones in
the arc, their branching axons aid in distributing the excitation; and
so we get a big movement in response to a minute, though intense
stimulus.

But the response is not simply big; it is definite, coordinated,
representing team work on the part of the muscles as distinguished
from indiscriminate mass action. That means selective distribution of
the nerve current. The axons of the sensory and central neurones do
not connect with any and every motor neurone indiscriminately, but
link up with selected groups of motor neurones, and thus harness
together teams that will work in definite ways, producing {39} flexion
of a limb in the case of one such team, and extension in the case of
another. Every reflex has its own team of motor neurones, harnessed
together by its outfit of sensory and central neurones. The same motor
neurone may however be harnessed into two or more such teams, as is
seen from the fact that the same muscle may participate in different
reflex movements; and for a similar reason we believe that the same
sensory neurone may be utilized in more than one reflex arc.

[Illustration: Fig. 11.--Cooerdination brought about by the branching
of the axon of a central neurone. (Figure text: sensory, central,
motor)]

The most distinctive part of any reflex arc is likely to be its
central neurones, which are believed to play the chief part in
cooerdination, and in determining the peculiarities of any given
reflex, such as its speed and rhythm of action.

Reactions in General

Though the reflex is simple by comparison with voluntary movements, it
is not the simplest animal reaction, for it is cooerdinated and depends
on the nervous system, while the simplest animals, one-celled animals,
have no nervous system, any more than they have muscles or