Among simple multicellular animals, such as sea-anemones, we find the
beginnings of reflex actions, and a considerable part of the behaviour
of the lower animals is reflex. That is to say, there are laid down in
the animal in the course of its development certain pre-arrangements of
nerve-cells and muscle-cells which secure that a fit and proper answer
is given to a frequently recurrent stimulus. An earthworm half out of
its burrow becomes aware of the light tread of a thrush's foot, and
jerks itself back into its hole before anyone can say "reflex action."
What is it that happens?
Certain sensory nerve-cells in the earthworm's skin are stimulated by
vibrations in the earth; the message travels down a sensory nerve-fibre
from each of the stimulated cells and enters the nerve-cord. The sensory
fibres come into vital connection with branches of intermediary,
associative, or communicating cells, which are likewise connected with
motor nerve-cells. To these the message is thus shunted. From the motor
nerve-cells an impulse or command travels by motor nerve-fibres, one
from each cell, to the muscles, which contract. If this took as long to
happen as it takes to describe, even in outline, it would not be of much
use to the earthworm. But the motor answer follows the sensory stimulus
almost instantaneously. The great advantage of establishing or
enregistering these reflex chains is that the answers are practically
ready-made or inborn, not requiring to be learned. It is not necessary
that the brain should be stimulated if there is a brain; nor does the
animal will to act, though in certain cases it may by means of higher
controlling nerve-centres keep the natural reflex response from being
given, as happens, for instance, when we control a cough or a sneeze on
some solemn occasion. The evolutionary method, if we may use the
expression, has been to enregister ready-made responses; and as we
ascend the animal kingdom, we find reflex actions becoming complicated
and often linked together, so that the occurrence of one pulls the
trigger of another, and so on in a chain. The behaviour of the
insectivorous plant called Venus's fly-trap when it shuts on an insect
is like a reflex action in an animal, but plants have no definite
nervous system.