explicitly, "Two membranes enclose the brain; that about the skull
is the stronger; the inner membrane is slighter than the outer one." And
further, it should be noted that he describes the latter membrane as a
vascular one. The fact of the brain substance being insensible to
mechanical irritation was known to Aristotle, and may have been learnt
from the practice of Hippocrates. Lastly, it should be remembered
that--though this may have been but a lucky guess on Aristotle's
part--the relative weight of brain to the entire body has been shown,
with few exceptions, to be greater in man than in any other animal.

In describing the _heart_ Aristotle says: "The heart lies about the
centre of the body, but rather in its upper than in its lower half, and
also more in front than behind.... In man it inclines a little towards
the left, so that it may counterbalance the chilliness of that side. It
is hollow, to serve for the reception of the blood; while its wall is
thick, that it may serve to protect the source of heat. For here, and
here alone, in all the viscera, and in fact in all the body, there is
blood without blood-vessels, the blood elsewhere being always contained
within vessels. The heart is the first of all the parts of the body to
be formed, and no sooner is it formed than it contains blood.... For no
sooner is the embryo formed than its heart is seen in motion like a
living creature, and this before any of the other parts. The heart is
abundantly supplied with sinews.... In no animal does the heart contain
a bone, certainly in none of those that we ourselves have inspected,
with the exception of the horse and a certain kind of ox. In animals of
great size the heart has three cavities; in smaller animals it has two;
and in all it has at least one."

It will be observed that here Aristotle so correctly describes the
position of the human heart as to render it probable that he is speaking
from actual inspection; although man is not the only animal in which the
heart is turned towards the left. In contrasting the heart with the
other viscera he appears to have overlooked the existence of the
coronary vessels, and to have imagined that the nutrition of the heart
was effected directly by the blood in its cavities. Although the heart
is not really the first part to appear, the observation of its very
early appearance in the embryo, which he treats more fully elsewhere,[8]
is alone enough to establish his reputation as an o