the whole mass, and capable of contraction in small tracts,
so that small areas of the brain may, at any given moment, become
bloodless, while other parts of the brain may simultaneously become
highly congested. Now, if the brain or any part of it be deprived of the
circulation of blood through it, or be rendered partially bloodless, or
if it be excessively congested and overloaded with blood, or if it be
subjected to local pressure, the part of the brain so acted upon ceases
to be capable of exercising its functions. The regularity of the action
of the brain and the sanity and completeness of the thought which is one
of the functions of its activity depend upon the healthy regularity of
the quantity of blood passing through all its parts, and upon the
healthy quality of the blood so circulating. If we press upon the
carotid arteries which pass up through the neck to form the arterial
circle of Willis, at the base of the brain, within the skull--of which I
have already spoken, and which supplies the brain with blood--we quickly,
as every one knows, produce insensibility. Thought is abolished,
consciousness lost. And if we continue the pressure, all those automatic
actions of the body, such as the beating of the heart, the breathing
motions of the lungs, which maintain life and are controlled by the
lower brain centers of ganglia, are quickly stopped and death ensues.

"We know by observation in cases where portions of the skull have been
removed, either in men or in animals, that during natural sleep the
upper part of the brain--its convoluted surface, which in health and in
the waking state is faintly pink, like a blushing cheek, from the color
of the blood circulating through the network of capillary
arteries--becomes white and almost bloodless. It is in these upper
convolutions of the brain, as we also know, that the will and the
directing power are resident; so that in sleep the will is abolished and
consciousness fades gradually away, as the blood is pressed out by the
contraction of the arteries. So, also, the consciousness and the
directing will may be abolished by altering the quality of the blood
passing through the convolutions of the brain. We may introduce a
volatile substance, such as chloroform, and its first effect will be to
abolish consciousness and induce profound slumber and a blessed
insensibility to pain. The like effects will follow more slowly upon the
absorption of a drug, such as opium; or we ma