I have been careful here to state that combined overwork of mind and
body is doubly mischievous, because nothing is now more sure in hygienic
science than that a proper alternation of physical and mental labor is
best fitted to insure a lifetime of wholesome and vigorous intellectual
exertion. This is probably due to several causes, but principally to the
fact that during active exertion of the body the brain cannot be
employed intensely, and therefore has secured to it a state of repose
which even sleep is not always competent to supply. There is a Turkish
proverb which occurs to me here, like most proverbs, more or less true:
"Dreaming goes afoot, but who can think on horseback?" Perhaps, too,
there is concerned a physiological law, which, though somewhat
mysterious, I may again have to summon to my aid in the way of
explanation. It is known as the law of Treviranus, its discoverer, and
may thus be briefly stated: Each organ is to every other as an excreting
organ. In other words, to insure perfect health, every tissue, bone,
nerve, tendon, or muscle should take from the blood certain materials
and return to it certain others. To do this every organ must or ought to
have its period of activity and of rest, so as to keep the vital fluid
in a proper state to nourish every other part. This process in perfect
health is a system of mutual assurance, and is probably essential to a
condition of entire vigor of both mind and body.

It has long been believed that maladies of the nervous system are
increasing rapidly in the more crowded portions of the United States;
but I am not aware that any one has studied the death-records to make
sure of the accuracy of this opinion. There can be no doubt, I think,
that the palsy of children becomes more frequent in cities just in
proportion to their growth in population. I mention it here because, as
it is a disease which does not kill but only cripples, it has no place