us impulses generated in the
receptive organs by environmental agents as stimuli. In other words,
whatever the nature of the agent, its result on the receptive organs
enters the central nervous organ as a nervous impulse, and all segments
of the central nervous organ receive impulses so generated. Further, it
is not known that nervous impulses present qualitative differences among
themselves. It is with these impulses that the central nervous organ
whether spinal cord or brain has to deal.

_Material and Psychical Signs of Cerebral Activity._--In the central
nervous organ the action resulting from entrant impulses has issue in
three kinds of ways. The reaction may die out, be suppressed, and so far
as discoverable lead to nothing; or the impulses may evoke effect in
either or both of two forms. Just as from the receptive organs, nerves
lead into the central nervous organ, so conversely from the central
organ other nerves, termed _efferent_, lead to various organs of the
body, especially glands and muscles. The reaction of the central nervous
organ to impulses poured into it commonly leads to a discharge of
impulses from it into glands and muscles. These centrifugal impulses
are, so far as is known, qualitatively like the centripetal impulses. On
reaching the glands and muscles they influence the activity of those
organs. Since those organs are therefore the mechanisms in which the
ultimate effect of the nervous reaction takes place, they are often
termed from this point of view _effector organs_. A change ensuing in
effector organs is often the only sign an observer has that a nervous
reaction has occurred, unless the nervous system under observation be
the observer's own.

If the observer turns to his own nervous system for evidence of
reaction, he meets at once in numberless instances with _sensation_ as
an outcome or sign of its reaction. This effect he cannot show to any
being beside himself. He can only describe it, and in describing it he
cannot strictly translate it into any term of material existence. The
unbridged gulf between sensation and the changes produced in effector
organs necessitates a separate handling of the functions of the nervous
system according as their office under consideration is sensation or
material effect. This holds especially in the case of the brain, and for
the following reasons.

_Psychosis and the Fore-Brain._--Hippocrates wrote, "It is through the
brain that we become mad, that d