t a certain way,
after a certain set of circumstances; that they last about so long, that
they are made worse by such and such things, that they are helped by
other things, and that they generally get better after a good night's
sleep.

One of the commonest causes of this group of recurrent and self-limited
headaches is fatigue, whether bodily, mental, or emotional. This was
long an apparent stumbling-block in the way of a poison theory of
headache, but now it is one of its best illustrations. Physiologists
years ago discovered that what produced not merely the sensation but
also the fact of fatigue, or tiredness, was the accumulation in the
muscles or nerves of the waste-products of their own activities. Simply
washing these out with a salt solution would start the utterly fatigued
muscle contracting again, without any fresh nourishment or even period
for rest. It has become an axiom with physiologists that fatigue is
simply a form of self-poisoning, or, as they sonorously phrase it,
autointoxication. One of the reasons why we are so easily fatigued when
we are already ill, or, as we say, "out of sorts," is that our tissues
are already so saturated with waste-products or other poisons that the
slightest addition of the fatigue poisons is enough to overwhelm them.
This also explains why our pet variety of headache, which we may have
clearly recognized to be due to overwork or overstrain of some sort,
whether with eye, brain, or muscles, is so much more easily brought on
by such comparatively small amounts of over-exertion whenever we are
already below par and out of sorts. People who are "born tired," who are
neurasthenic and easily fatigued and "ached," are probably in a chronic
state of self-poisoning due to some defect in their body-chemistry.
Further, the somewhat greater frequency and acuteness of headache in
brain workers--although the difference between them and muscle workers
in this regard has been exaggerated--is probably due in part to the
greater sensitiveness of their nerves; but more so to the curious fact,
discovered in careful experiments upon the nervous system, that the
fatigue products of the nerve-cells are the deadliest and most powerful
poisons produced in the body. Hence some brain workers can work only a
few half-hours a day, or even minutes at a time; for instance, Darwin,
Spencer, and Descartes.

A very frequent cause of these habitual headaches, really a subdivision
of the great fatigue grou