Every kind of mental activity uses up the brain energy of the cells. To think clearly, plenty of healthy blood must be supplied to the cells. In order to make healthy blood, there must be an abundance of fresh air supplied to the lungs, and a vigorous heart to pump it up to the brain. It has been demonstrated that, during intellectual work or emotional feeling, there is an increased supply of blood to the brain, which may become more or less congested, and that there is an actual rise of temperature; whereas during periods of relaxation, rest, or fatigue, the brain is pale and anemic. The brain-cells generally, but particularly those cells involved in mental activity, are of such a nature and constitution that they cannot rest absolutely during the waking hours. They may act slowly or with great rapidity; different brains have different capacities for energizing, both in regard to speed and force; and, further, the brain may be pushed to work greatly in excess of its normal activity, just as an engine may be allowed to go at the rate of 50 or 60 miles an hour, or may be pushed to go at the rate of 100 miles an hour. In both cases the danger resulting from speeding are greatly in excess of going at the normal rate of speed. In any case, the continuous brain action implies the necessity for continuous repair. The only complete physiologic rest which the brain enjoys is during sleep, when the process of repair goes on most rapidly; during this period the brain-cells absorb their nourishment from the blood in excess of their needs, and so lay up a store of energy for the waking hours. =Habit and Automatism.=—It is one of the innate qualities of every tissue and of every organ in the body, that when any vital action is performed, any vital process gone through with, it is easier to do it the second time, and the continuous exercise of the action makes the performances more and more easy, until they become automatic. The physiologic basis of habits consists of the plasticity of the nerve substance, and in the capacity of nerve substance to receive and retain impressions. There results the certainty that the nervous system will act again more easily in those ways in which it has already acted.