y that changes may be produced in the mind in other ways than
through the regular channels of sense. For example, we influence the
mind when we drink too much tea or coffee, not to mention the greater
changes of the same kind which are produced in the mind of the drinker
of too much alcohol or other poisonous substances. All the methodical
means of procedure by which the psychologist produces effects of this
kind by changing the condition or functions of the body within itself
belong to Physiological Psychology. So he modifies the respiration,
changes the heart beat, stimulates or slows the circulation of the
blood, paralyzes the muscles, etc. The ways of procedure may be
classified under a few heads, each called a method.

1. _Method of Extirpation._--This means simply the cutting away of a
part of the body, so that any effect which the loss of the part makes
upon the mind may be noted. It is used especially upon the brain.
Pieces of the brain, great or small--indeed, practically the whole
brain mass--may be removed in many animals without destroying life.
Either of the cerebral hemispheres entire, together with large
portions of the other, may be taken from the human brain without much
effect upon the vital processes, considered as a whole; the actual
results being the loss of certain mental functions, such as sight,
hearing, power of movement of particular limbs, etc., according to the
location of the part which is removed. Many of the facts given below
under the heading of Localization were discovered in this way, the
guiding principle being that if the loss of a function follows the
removal of a certain piece of the brain, then that portion of the
brain is directly concerned in the healthy performance of that
function.

2. _Method of Artificial Stimulation._--As the term indicates, this
method proceeds by finding some sort of agent by which the
physiological processes may be started artificially; that is, without
the usual normal starting of these processes. For example, the
physician who stimulates the heart by giving digitalis pursues this
method. For psychological purposes this method has also been fruitful
in studying the brain, and electricity is the agent customarily used.
The brain is laid bare by removing part of the skull of the animal,
and the two electrodes of a battery are placed upon a particular point
of the brain whose function it is wished to determine. The current
passes out along the nerves whic