, or lymphatic vessels, appear as
very delicate threads throughout their whole length. The nerves, on the
contrary, do not penetrate the substance of the brain; they abut upon
its surface only; before reaching it they lose their elasticity and
solidity, and the extremities of the nerves which are nearest to the
brain are soft, and nearly mucilaginous. From this exposition, in which
there is nothing hypothetical, it appears that the brain, which is
nourished by the lymphatic arteries, does in its turn provide
nourishment for the nerves, and that we must regard these as a kind of
vegetation which rises as trunks and branches from the brain, and become
subsequently subdivided into an infinite number, as it were, of twigs.
The brain is to the nerves what the earth is to plants: the last
extremities of the nerves are the roots, which with every vegetable are
more soft and tender than the trunk or branches; they contain a ductile
matter fit for the growth and nourishment of the nervous tree or fibre;
they draw the ductile matter from the substance of the brain itself, to
which the arteries are continually bringing the lymph that is necessary
to supply it. The brain, then, instead of being the seat of the
sensations, and the originator of perception, is an organ of secretion
and nutrition only, though a very essential organ, without which the
nerves could neither grow nor be maintained.

"This organ is greater in man, in quadrupeds, and in birds, because the
number or bulk of the nerves is greater in these animals than in fishes
or insects, whose power of perception is more feeble, for this very
reason, that they have but a small brain; one, in fact, that is
proportioned to the small quantity of nerves which that brain must
support. Nor can I omit to state here that man has not, as has been
pretended by some, a larger brain than has any other animal; for there
are apes and cetacea which have more brain than man in proportion to the
volume of their bodies--another fact which proves that the brain is
neither the seat of sensations nor the originator of perception, since
in that case these animals would have more sensations and perception
than man.

"If we consider the manner in which plants derive their nourishment, we
shall find that they do not draw up the grosser parts either of earth or
water; these parts must be reduced by warmth into subtle vapours before
the roots can suck them up into the plant. In like manner the nutri