" and erroneously describing the motion of the blood, not to
the contraction of the walls of the heart, but to the heat which he
supposes to be generated there, he adds:--

"This motion, which I have just explained, is as much the necessary
result of the structure of the parts which one can see in the
heart, and of the heat which one may feel there with one's fingers,
and of the nature of the blood, which may be experimentally
ascertained; as is that of a clock of the force, the situation, and
the figure, of its weight and of its wheels."

But if this apparently vital operation were explicable as a simple
mechanism, might not other vital operations be reducible to the same
category? Descartes replies without hesitation in the affirmative.

"The animal spirits," says he, "resemble a very subtle fluid, or a
very pure and vivid flame, and are continually generated in the
heart, and ascend to the brain as to a sort of reservoir. Hence
they pass into the nerves and are distributed to the muscles,
causing contraction, or relaxation, according to their quantity."

Thus, according to Descartes, the animal body is an automaton, which is
competent to perform all the animal functions in exactly the same way as
a clock or any other piece of mechanism. As he puts the case himself:--

"In proportion as these spirits [the animal spirits] enter the
cavities of the brain, they pass thence into the pores of its
substance, and from these pores into the nerves; where, according
as they enter, or even only tend to enter, more or less, into one
than into another, they have the power of altering the figure of
the muscles into which the nerves are inserted, and by this means
of causing all the limbs to move. Thus, as you may have seen in the
grottoes and the fountains in royal gardens, the force with which
the water issues from its reservoir is sufficient to move various
machines, and even to make them play instruments, or pronounce
words according to the different disposition of the pipes which
lead the water.

"And, in truth, the nerves of the machine which I am describing may
very well be compared to the pipes of these waterworks; its muscles
and its tendons to the other various engines and springs which seem
to move them; its animal spirits to the water which impels them, of
which the heart is the