Conditions like Fatigue
Matters are complicated by the fact that physical conditions other than ordinary use produce fatigue-like effects in nerve cells. Excitement of any kind rapidly changes the condition of nerve cells, and sometimes foreign chemical substances get into the blood, as in fever or infection, and produce a condition that is in effect the same as fatigue.
Still further, as a fact of large importance in determining capacity for work, the nerve cells pass each day through a kind of internal cycle of conditions. At certain hours their condition is such that they transmit nervous impulses freely, and work is easy; at other hours work drags because the nerve cells are not prepared to be active; their internal chemical condition is such as to obstruct transmission of impulses. Thus, one is usually very energetic in the middle of the forenoon, but is logy at noon and sleepy at a late hour in the afternoon. Marked individual differences appear, making this statement merely a general statement. Furthermore, personal habits can be changed to some extent through the adoption of new habits of life.
Finally, there are all sorts of pathological conditions which profoundly affect the life and action of nerve cells. Anæmia and malnutrition may render nerve cells utterly incapable of continued action.