organ to the nerve center, and another, the motor nerve, runs from the
center to the muscle; and the only connection between the sense organ
and the muscle is this roundabout path through the nerve center. The
path consists of three parts, sensory nerve, center, and motor nerve,
but, taken as a whole, it is called the _reflex arc_, both the words,
"reflex" and "arc", being suggested by the indirectness of the
connection.

[Illustration: Fig. 1.--The connection from the back of the hand,
which is receiving a stimulus, and the arm muscle which makes the
response. The nerve center is indicated by the dotted lines.]

The _nervous system_ resembles a city telephone system. What passes
along the nerve is akin to the electricity that {28} passes along the
telephone wire; it is called the "nerve current", and is electrical
and chemical in nature.

[Illustration: Fig. 2.--(From Martin's "Human Body.") General view of
the nervous system, showing brain, cord, and nerves.]

All nerve connections, like the great majority of telephone
connections, are effected through the centers, called "centrals" in
{29} the case of the telephone. Telephone A is connected directly with
the central, telephone B likewise, and A and B are indirectly
connected, through the central switchboard. That is the way it is in
the nervous system, with "nerve center" substituted for "central", and
"sense organ" and "muscle or gland" for "telephones A and B."

[Illustration: Fig. 3.--Location of the cord, cerebrum and cerebellum.
The brain stem continues the cord upward into the skull cavity.
(Figure text: cerebrum, cerebellum, cord, tongue)]

The advantage of the centralized system is that it is a _system_,
affording connections between any part and any other, and unifying the
whole complex organism.

The _nerve centers_ are located in the brain and spinal cord. The
brain lies in the skull and the cord extends from the brain down
through a tube in the middle of the {30} backbone. Of the brain many
parts can be named, but for the present it is enough to divide it into
the "brain stem", a continuation of the spinal cord up along the base
of the skull cavity, and the two great outgrowths of the brain stem,
called "cerebrum" and "cerebellum". The spinal cord and brain stem
contain the lower or reflex centers, while the cerebellum, and
especially the cerebrum, contain the "higher centers". The lower
centers are directly connected by nerves with t