in may be advantageously used, is
modified by many circumstances._ The power of the brain in different
persons to endure action, is various. This is modified by its primary
character; by development and age; by habits of action; by the health
of the cerebral organ and general system; by the moral feelings and
other conditions.

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825. Why should we engage in intense study in the early part of the
day? 826. What persons require the most sleep? Why? 827. What is said
relative to the length of time that the brain can be advantageously
used? Give a condition that modifies the amount of mental labor.

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828. The primary physical organization of some individuals is such,
that they are enabled to endure with impunity an amount of mental
labor that would disorder, if not destroy functionally, the cerebral
organ of others differently constituted. Napoleon Bonaparte was of
this number. There can be no fixed period for mental labor, that may
be adopted as a rule for all persons whose systems are maturely
developed. Much less is there a proper definite period for study, that
is applicable to all children.

_Observation._ The practice of retaining pupils of all ages, from five
to twenty years, in the school-room the same period of time, for the
purpose of study, is not predicated upon any law of physiology. An
exercise of three hours, with one or two recesses of ten minutes each,
may profit the eldest class; two hours with a recess of ten minutes,
the middle class; while one hour, or one hour and a half, with one
recess, would be as long a period as the youngest pupils should be
retained in the study-room at one session.

829. A person who is accustomed to muscular exertion will endure a
longer period of physical toil than one who is not inured to it. So it
is with mental labor. If the brain has been habituated to mental
action and profound study, it will not be so soon fatigued as when not
accustomed to such exertions; consequently, an amount of mental labor
may be performed with impunity at one time, that would exhaust and
cause serious disease of the cerebral organ at another.

_Observation._ Persons that commence a course of study at a late
period in life, frequently evince their zeal at the commencement by
poring over their books twelve or more hours each day. The progress of
such students is soon arrested by physical and mental depression. In
such instances, it would