prepares chyme and chyle. For similar reasons, the brain
should never be used during, or immediately after, violent muscular
exercise. For the motor nerves are in this respect on a par with the
sensory nerves; the pain felt when a limb is wounded has its seat in
the brain; and, in the same way, it is not really our legs and arms
which work and move,--it is the brain, or, more strictly, that part of
it which, through the medium of the spine, excites the nerves in the
limbs and sets them in motion. Accordingly, when our arms and legs
feel tired, the true seat of this feeling is in the brain. This is why
it is only in connection with those muscles which are set in motion
consciously and voluntarily,--in other words, depend for their action
upon the brain,--that any feeling of fatigue can arise; this is not
the case with those muscles which work involuntarily, like the heart.
It is obvious, then, that injury is done to the brain if violent
muscular exercise and intellectual exertion are forced upon it at the
same moment, or at very short intervals.

What I say stands in no contradiction with the fact that at the
beginning of a walk, or at any period of a short stroll, there often
comes a feeling of enhanced intellectual vigor. The parts of the brain
that come into play have had no time to become tired; and besides,
slight muscular exercise conduces to activity of the respiratory
organs, and causes a purer and more oxydated supply of arterial blood
to mount to the brain.

It is most important to allow the brain the full measure of sleep
which is required to restore it; for sleep is to a man's whole nature
what winding up is to a clock.[1] This measure will vary directly with
the development and activity of the brain; to overstep the measure is
mere waste of time, because if that is done, sleep gains only so much
in length as it loses in depth.[2]

[Footnote 1: _Of. Welt als Wille und Vorstellung_, 4th Edition. Bk.
II. pp. 236-40.]

[Footnote: 2: _Cf. loc: cit_: p. 275. Sleep is a morsel of death
borrowed to keep up and renew the part of life which is exhausted by
the day--_le sommeil est un emprunt fait a la mort_. Or it might be
said that sleep is the interest we have to pay on the capital which is
called in at death; and the higher the rate of interest and the
more regularly it is paid, the further the date of redemption is
postponed.]

It should be clearly understood that thought is nothing but the
organic functio