he sense organs, glands
and muscles, while the higher centers have direct connections with the
lower and only through them with the sense organs, glands and muscles.
In other words, the sensory nerves run into the cord or brain stem,
and the motor nerves run out of these same, while interconnecting
nerve strands extend between the lower centers in the cord and brain
stem and the higher centers in the cerebrum and cerebellum.

The spinal cord contains the reflex centers for the limbs and part of
the trunk, and is connected by sensory and motor nerves with the limbs
and trunk. The brain stem contains the reflex centers for the head and
also for part of the interior of the trunk, including the heart and
lungs, and is connected with them by sensory and motor nerves. The
nerve center that takes part in the flexion reflex of the foot is
situated in the lower part of the cord, that for the similar reflex of
the hand lies in the upper part of the cord, that for breathing lies
in the lower or rear part of the brain stem, and that for winking lies
further forward in the brain stem.

Big movements, such as the combined action of all four legs of an
animal in walking, require cord and brain stem to work together, and
throw into relief what is really true even of simpler reflexes, namely
that a reflex is a _coordinated_ movement, in the sense that different
muscles cooperate in its execution.

{31}

Internal Construction of the Nerves and Nerve Centers

We shall understand nerve action better if we know something of the
way in which the nervous system is built. A nerve is not to be thought
of as a unit, nor are the brain and cord to be thought of as mere
masses of some peculiar substance.

[Illustration: Fig. 4.--A motor nerve cell from the spinal cord,
highly magnified. (Figure text: dendrites, cell body, axon,
termination of axon in muscle)]

A nerve is a bundle of many slender insulated threads, just as a
telephone cable, running along the street, {32} is a bundle of many
separate wires which are the real units of telephonic communication. A
nerve center, like the switchboard in a telephone central, consists of
many parts and connections.

The whole nervous system is essentially composed of _neurones_. A
neurone is a nerve cell with its branches. Most nerve cells have two
kinds of branches, called the _axon_ and the _dendrites_.

The nerve cell is a microscopic speck of living matter. Its dendrites
are short tree-