In our search for means of preventing disease we have been led, as we see, beyond the province of medicine and into that of education. But the connection between these two fields, from our standpoint, extends much further, as we shall find. Though muscular exercise be carried to the point of perfection, and the surrounding conditions leave nothing to be desired, health is not assured; for, should the expenditure of energy be too great, there will still be marked interference with development. Hence, the expenditure as well as the development of energy must be considered.
Now, the energy is expended by the organs in the performance of their functions. Though ultimately derived from the food, its proximate source is, at least in great part, the tissues, which, by undergoing combustion, furnish the required energy; whether it be all derived in this way, or whether it be in part supplied immediately by the blood, is a matter which has no influence upon our problem. It is essential, however, to bear in mind that the amount of energy which can be developed in a given space of time is limited to the quantity of food digestible during this period, and, if it be expended more rapidly than it is thus supplied, the functions are performed at the expense of the tissues. If, therefore, we desire to guard the system against waste and allow the organs to develop properly, we must, as far as possible, limit functional activity. There are, however, only two functions over which we can exert a direct voluntary influence, namely, the muscular and the mental. But as there is ample compensation for the energy expended in muscular activity while there is none, in the physical sense, for that used in mental action, it is clear that if we desire to economize we must do so by exercising control over mental labor. That the energy expended by the brain during its activity is derived from the same source as that required for the performance of the other functions is, to-day, a generally accepted fact.
But, in order that it may not interfere with physical development, mental labor must be regulated both quantitatively and qualitatively.
In regard to quantity, the number of school hours must not be excessive; and introduction into school life should be gradual, so that the labor may be in proportion to the age and strength of the child. In Germany this rule is observed, the children beginning with about sixteen hours per week, to which two are added every year until the fifth or sixth, when the maximum is reached. The amount of work required at school, and in the preparation of lessons, likewise needs careful consideration.
From the qualitative standpoint the methods of instruction play an important part. When the laws of psychology are observed, the mind being treated in accordance with its nature, the