ngle nerve fibers are fine, thread-like cells. They are so small as to
be invisible to the naked eye. Some of them are so minute that it would
take twenty thousand of them laid side by side to measure an inch. Every
nerve fiber in the human body forms one of a series of connecting links
between some central nerve cell in the brain or spinal cord on the one
hand and some bodily tissue on the other.

All nerves originating in the brain may be divided into two classes
according as they carry currents to the brain or from it. Those carrying
currents to the brain are called _sensory_ nerves, or nerves of
sensation; those carrying currents from the brain are called _motor_
nerves, or nerves of motion.

[Sidenote: Couriers of Action]

Among the sensory nerves are the nerves of consciousness; that is, the
nerves whereby we receive sense impressions from the external world.
These include the nerves of touch, sight, pain, hearing, temperature,
taste and smell. Motor nerves are those that carry messages from the
brain and spinal cord on the one hand to the muscles on the other. They
are the lines along which flash all orders resulting in bodily
movements.

[Sidenote: Nerve Systems]

Another broad division of nerves is into two great nerve systems. There
are the _cerebro-spinal_ system and the _sympathetic_ system. The first,
the cerebro-spinal system, includes all the nerves of _consciousness_
and of _voluntary action_; it includes all nerves running between the
brain and spinal cord on the one hand and the voluntary muscles on the
other. The second, the sympathetic nerve system, consists of all the
nerves of the unconscious or functional life; it therefore includes all
nerves running between the brain and sympathetic or involuntary nerve
centers on the one hand and the involuntary muscles on the other.

Every bodily movement or function that you can start or stop at will,
even to such seemingly unconscious acts as winking, walking, etc., is
controlled through the cerebro-spinal system. All other functions of the
body, including the great vital processes, such as heart pulsation and
digestion, are performed unconsciously, are beyond the direct control of
the will, and are governed through the sympathetic nerve system.

[Sidenote: Organs of Consciousness and Subconsciousness]

It is obvious that the cerebro-spinal nerve system is the organ of
consciousness, the apparatus through which the mind exercises its
conscious and v