p, is eye-strain. This is due to an abnormal or
imperfect shape of the eye, which is usually present from birth. Hence,
the only possible way of correcting it is by the addition to the
imperfect eye of carefully fitted lenses or spectacles which will
neutralize this mechanical defect. To put it very roughly, if the eye is
too flat to bring the light-rays to a focus upon the retina, which is
far the commonest condition (the well-known "long sight," or hyperopia),
we put a plus or bulging glass before the eye and thus correct its
shape. But if the eye is too round and bulging, producing the familiar
"short sight," or myopia, we put a minus or concave lens before the eye,
and thus bring it back to the normal. By a curious paradox, however, it
often happens that the headache due to eye-strain is caused not by the
grosser defects, such as interfere with vision so seriously as
absolutely to demand the wearing of glasses to see decently, but from
slighter and more irregular degrees and kinds of misshapenness in the
eye, most of which fall under the well-known heading of astigmatism.
These interfere only slightly with vision, but keep the eye perpetually
on the strain, on a twist, as it were, rasping the entire nervous system
into a state of chronic irritation. Our motto now, in all cases of
chronic headache, is, first examine the patient's habits of life, next
his eyes.

Many forms of headache are really stomach-ache in disguise, due to
digestive disturbances, the absorption of poisons from the food-tube,
whether from tainted, spoiled, or decayed foods, as in the now familiar
ptomaine poisoning, or from imperfect processes of digestion. The
immediate effect, however, of diet in the causation of headache is not
so great as we once believed. We have no adequate basis for believing
that any particular kinds or amounts of food are especially likely to
produce either headache or what we might call the headache habit, except
in so far as they upset the digestion. In a certain number of
susceptible individuals, however, it will be found that some particular
kind of food, often perfectly wholesome and harmless in itself, will
bring on an attack of headache whenever it is indulged in. Very
frequently the disturbances of digestion which are put down as the
_cause_ of a headache are only _symptoms_ of some general constitutional
lack of balance, as eye-strain or neurasthenia, which is the cause of
both these discomforts. Far fewer headac