tired of the latter.  What we really mean is that we would rather do something else.  These obtruding ideas are often introduced into our minds by other people who tell us that we have worked long enough and ought to come and play, and though we may not have felt tired up to this point, still 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.”