time to the external vibration, or, as the older men called it,
'stimulus' from without, is also undergoing rest and repair, so that,
when it comes again into work, it may receive better the impressions
it may have to gather up, and influence more effectively the muscles
it may be called upon to animate, direct, control."[11] An American
observer and physiologist, Dr. William A. Hammond, confirms the views
of his English colleague. He tells us that "the state of general
repose which accompanies sleep is of especial value to the organism,
in allowing the nutrition of the nervous tissue to go on at a greater
rate than its destructive metamorphosis." In another place he adds,
"For the brain, there is no rest except during sleep." And, again, he
says, "The more active the mind, the greater the necessity for sleep;
just as with a steamer, the greater the number of revolutions its
engine makes, the more imperative is the demand for fuel."[12] These
statements justify and explain the instinctive demand for sleep. They
also show why it is that infants require more sleep than children, and
children than middle-age folk, and middle-age folk than old people.
Infants must have sleep for repair and rapid growth; children, for
repair and moderate growth; middle-age folk, for repair without
growth; and old people, only for the minimum of repair. Girls, between
the ages of fourteen and eighteen, must have sleep, not only for
repair and growth, like boys, but for the additional task of
constructing, or, more properly speaking, of developing and perfecting
then, a reproductive system,--the engine within an engine. The bearing
of this physiological fact upon education is obvious. Work of the
school is work of the brain. Work of the brain eats the brain away.
Sleep is the chance and laboratory of repair. If a child's brain-work
and sleep are normally proportioned to each other, each night will
more than make good each day's loss. Clear heads will greet each
welcome morn. But if the reverse occurs, the night will not repair the