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.