same holds true if the physical source lies in my own body, if perhaps
my tooth begins to ache, although no expectation preceded it.

In the same way it seems a matter of course that mind and body are
connected wherever an action is performed. I have the will to grasp for
the book before me, and obediently my arm performs the movement; the
muscles contract themselves, the whole physical apparatus comes into
motion through the preceding mental fact. The same holds true where no
special will act arouses the muscles. If a thought is in my mind and it
discharges itself in appropriate words, those words are after all as
physical facts the movements of lips and tongue and vocal cords and
chest; in short, a whole system of physical responses has set in through
a mental experience. But the same thought may be the starting-point for
many other bodily changes; it may make me blush, and that means that
large groups of blood-vessels become dilated; or I may get pale, the
blood-vessels are contracted. Or I may cry, the lachrymal gland is
working; or it may spoil my appetite, the membranes of my stomach cease
to produce; or my muscles may tremble, or my skin may perspire; in
short, my whole organism may resound with mental excitement which some
words may set up.

But it is not only the impression of outer stimuli and the expression of
inner thoughts in which mind and body come together. Daily life teaches
us, for instance, how our mental states are dependent upon most various
bodily influences. If the temperature of the blood is raised in fever,
the mental processes may go over into far-reaching confusion; if hashish
is smoked, the mind wanders to paradise, and a few glasses of wine may
give a new mental optimism and exuberance; a cup of tea may make us
sociable, a dose of bromide may annihilate the irritation of our mind,
and when we inhale ether, the whole content of consciousness fades away.
In every one of these cases, the body received the chemical substance,
the blood absorbed and carried it to the brain, and the change in the
brain was accompanied by a change in the mental behavior. Even ordinary
sleep at night presents itself surely as a bodily state--the fatigued
brain cells demand their rest, and yet at the same time the whole mental
life becomes entirely changed. It is not difficult to carry over such
observations of daily life to the more exact studies of the
psychological laboratory and to examine with the subtle means