_c._ A further examination of the body reveals a central mental organism, the brain, composed of highly differentiated cells whose intelligence, as in the case of other cells, is commensurate with their functions. _d._ It reveals also a physical mechanism, the nervous system, peculiarly adapted to the communication of intelligence between the central governing intelligence and the subordinate cells. [Sidenote: An Instrument for Mental Dominance] _e._ The existence of this mind organism and this mechanism of intercommunication is additional evidence of the control and direction of bodily activities by _mental energy_. The facts to follow will not only demonstrate the truth of these propositions, but will disclose the existence within every one of us of a store of mental energies and activities of which we are entirely unconscious. The brain constitutes the organ of central governing intelligence, and the nerves are the physical means employed in bodily intercommunication. Brain and nerves are in other words the physical mechanism employed by the mind to dominate the body. [Sidenote: Gateways of Experience] Single 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.