"The medium uses the solar plexus as a mirror, and upon its sensitive nerves are reflected
pictures recorded in the invisible ethers.  Through the spleen (which is the gateway to the
etheric body) the medium permits decarnate intelligences to come into his spiritual
constitution, the result voices and other spirit manifestations.  Automatic writing is gained
by permitting the etheric arm of an outside intelligence to control temporarily the physical
arm of the medium.  This is not possible until the medium removes his own etheric double
from the arm, for two things cannot occupy the same place at the same time.  The process
of periodically separating the life forces from the physical arm is very dangerous, often
resulting in paralysis.  Mediumship is unnatural to man, while clairvoyance is the natural
result of growth and the unfolding of the spiritual nature.  There are a hundred mediums to
one clairvoyant, for the clairvoyant can become such only through self-mastery and
exertion of tremendous power; while the weaker, the more sickly, and the more nervous
and individual is, the better medium he makes.  The clairvoyant is unfolding his mind by
filling it with useful knowledge, while the first instruction given to the would-be medium is,
"Make your mind a blank."

The reason mediumship through the solar plexus is a retrogression may be summed up as
follows: The group spirits who control the destinies of the animal kingdoms govern their
charges through pictures thrown against the solar plexus, for the animal has no
self-conscious mind.  As a result, instead of thinking with its own brain it thinks with the
brain of the group spirit to whom it is attached by invisible magnetic cords.  These cords
convey their impressions and photograph them upon the sympathetic nervous system.  
Having no will of its own, the animal is incapable of combating these urges and
consequently obeys them implicitly.  Man governs himself through the cerebrospinal
nervous system because he has developed individuality and the sympathetic system no
longer rules him.  Opening himself to impulses through the solar plexus area, the medium is
thwarting his own growth by preventing the cerebrospinal nervous system from controlling
his destiny."

from - "The Occult Anatomy of Man"

~ Manley P. Hall