ll the suggestion is so strong that we immediately begin
to feel tired. Various social situations can arouse the same
suggestion. For example, as the clock nears quitting time, we feel that
we ought to be tired, so we allow ourselves to think we are.

Let us now examine the bodily conditions to see what fatigue is
objectively. "Physiologically it has been demonstrated that fatigue is
accompanied by three sorts of changes. First, poisons accumulate in the
blood and affect the action of the nervous system, as has been shown by
direct analysis. Mosso ... selected two dogs as nearly alike as
possible. One he kept tied all day; the other, he exercised until by
night it was thoroughly tired. Then he transfused the blood of the
tired animal into the veins of the rested one and produced in him all
the signs of fatigue that were shown by the other. There can be no
doubt that the waste products of the body accumulate in the blood and
interfere with the action of the nerve cells and muscles. It is
probable that these accumulations come as a result of mental as well as
of physical work.

"A second change in fatigue has been found in the cell body of the
neurone. Hodge showed that the size of the nucleus of the cell in the
spinal cord of a bee diminished nearly 75 per cent, as a result of the
day's activity, and that the nucleus became much less solid. A third
change that has been demonstrated as a result of muscular work is the
accumulation of waste products in the muscle tissue. Fatigued muscles
contain considerable percentages of these products. That they are
important factors in the fatigue process has been shown by washing them
from a fatigued muscle. As a result the muscle gains new capacity for
work. The experiments are performed on the muscles of a frog that have
been cut from the body and fatigued by electrical stimulation. When
they will no longer respond, their sensitivity may be renewed by
washing them in dilute alcohol or in a weak salt solution that will
dissolve the products of fatigue. It is probable that these products
stimulate the sense-organs in the muscles and thus give some of the
sensations of fatigue. Of these physical effects of fatigue, the
accumulation of waste products in the blood and the effects upon the
nerve cells are probably common both to mental and physical fatigue.
The effect upon the muscles plays a part in mental fatigue only so far
as all mental work involves some muscular activity."

By th