CIRCULATION.
678
reason to think that the motion of the blood
is first arrested in the pulmonary capillaries.
The state of our knowledge does not, it must
be confessed, permit us to offer a satisfactory
explanation of the cause of the above-men¬
tioned phenomena. We have already stated
reasons against regarding the stagnation of the
blood in the lungs in asphyxia as attributable
to a loss of the supposed vital power of motion
belonging to the blood in the capillary vessels :
and we think it quite as just to regard the stag¬
nation as the effect of over-stimulation and
constriction of the minute vessels of the lungs
by the dark blood, as to attribute it, in the
manner some have done, to the deficiency of
that stimulation which arterial blood, without
any good reason, is presumed by them to give
to the small vessels.
2. Circulation within the cranium. — The
limits of this essay do not permit us to do more
than allude very shortly to the nature of the
circulation within the cranium,—a subject, in
some respects, nearly related to the facts just
stated, and of great importance from the general
dependence of the state of the cerebral func¬
tions upon the quantity and force of blood which
flows through the brain.
The bloodvessels within the cranium are dif¬
ferently situated from those in other parts of
the body in this respect, that they are removed
from the influence of atmospheric pressure. In
consequence of the unyielding nature of the
skull, and its being closed on all sides, except¬
ing at the places where the nerves and blood¬
vessels pass through the bones, the cavity of the
skull must necessarily be equally full at all
times ; and the spinal canal is in the same pre¬
dicament.
The whole quantity of fluid or solid matter,
then, within the cavity of the cranium and
spinal canal must be always the same; or,
during the circulation just as much blood must
issue as enters it, and it is physically impossible
to increase or diminish the whole quantity con¬
tained in the brain by increased pressure, by
opening of an artery or vein or any other means.
It was shewn by various well devised experi¬
ments performed by the late Dr. Kellie,* that
in animals bled to death, while the rest of the
body was exsangueous, the brain retained its
usual appearance so long as the vault of the
cranium was entire, but that a perforation of the
skull, such as to allow the atmospheric pressure
to act upon the brain and bloodvessels of the
head, caused the evacuation of blood from the
head as from other parts of the body.
While the whole bulk of the contents of the
cranium, however, must necessarily remain the
same, yet the relative quantity of arterial and
venous blood may vary within a short space of
time, the pressure exerted by the blood in the
vessels may be greater or less according to cir¬
cumstances ; and there may occur within the
skull local determinations or partial distribu¬
tions of the blood. When from rupture of a
bloodvessel, inflammation, suppuration, or other
causes, blood, serum, or pus are effused into
* Edin. Med. Chirurg. Trans, vol. i.
the cavity of the cranium, the circulating blood
must be diminished in quantity ; when there is
any obstruction to the return of the blood by
the jugular veins, the pressure of the blood en¬
tering by the carotid artery is proportionally
greater; and when the arteries which supply
blood to the brain are obstructed, or the heart’s
action is less forcible than usual, the pressure
on the brain must be diminished in a corre¬
sponding degree.
In the natural state of the circulation the
pressure exerted by the blood circulating
through the cranium is subject to regular alter¬
nations of increase and decrease from the effect
of the heart’s action and the motions of respira¬
tion. When the brain of man or of animals is
exposed by the removal of a part of the skull, it
is seen to be slightly raised at the exposed part
at each arterial pulsation, and more perceptibly
during each expiration. The brain falls again
during each succeeding inspiration, but does
not sink below the level of the skull. These
motions may also be perceived at the fontanelles
of the infant’s head, where the bony parietes of
the skull are deficient. In the closed state of
the skull, for the reasons previously mentioned,
it is obvious that there can be no motions simi¬
lar to those observed in the brain when ex¬
posed, but nevertheless the brain must be more
forcibly pressed upon by the 'blood at these
times than at others. Haller, who had observed
these motions, conceived the depression during
inspiration to be caused simply by the ease
with which the blood enters the chest at that
time, and attributed the swelling of the brain
during expiration to the obstacle then offered
to the descent of the blood through the jugu¬
lar veins. It seems, however, probable that
the greater fulness of the arteries during
expiration may also contribute to raise the
brain at the time when the collapse of the
walls of the chest occurs : for Magendie ob¬
served, that when a ligature was put upon the
jugular vein, the blood which issued from this
vein by an aperture above the ligature, flowed
with greater force during expiration, shewing
that increased arterial pressure during expira¬
tion was continued through the capillaries into
the veins. Sign. Ravina, who made a very
extensive series of experiments upon these mo¬
tions, found that when the brain has been de¬
pressed during inspiration, it again swells,
although no expiration succeeds, but that when
raised during expiration, it does not again sink,
if inspiration does not follow.
3. Influence of varieties in the distribution
of arteries and veins upon the circulation.—As
connected with some of the above-mentioned
facts, and exerting a considerable influence in
modifying the circulation of the blood in parti¬
cular states of the animal economy, we may
here mention a few of the more remarkable
varieties in the distribution of the arteries and
veins, together with the uses they have been
supposed to serve in different animals. The
varieties of form in the larger arteries may be
considered under two heads; a, simple tor¬
tuosity ; and b, sudden division into many-
small branches.