706 
lation through the same channel as that of the 
dura mater, of the cranial bones, and of the 
eball: the internal jugular veins are the 
channel towards which the venous blood of the 
cranium tends. An obstacle, therefore, in both 
or either of these vessels must affect the entire 
venous system of the brain, or at least that of 
the corresponding hemisphere. A ligature tied 
tightly round the neck impedes the circulation, 
and may cause congestion of the brain. The 
bodies of criminals who have died by hanging 
exhibit great venous congestion, both of the 
walls and the contents of the cranium, in con- 
sequence of the strong compression to which 
the veins have been submitted. 
We have seen that, when the blood of one 
carotid artery is cut off, the parts usually sup- 
plied by it are mes to become exsangueous and 
softened ; and this is more especially the case 
if the vertebral be stopped up, or the circula- 
tion in it impeded. And it has been remarked, 
that these effects will follow the application of 
a ligature to either common carotid artery. 
Notwithstanding these facts, a doctrine has 
received very general assent, and the support 
of men of high reputation, which affirms that 
the absolute quantity of blood in the brain 
cannot vary, because that organ is incom- 
pressible, and is enclosed in a spheroidal case 
of bone, by which it is completely exempted 
from the pressure of the atmosphere. 
The cranium, however, although spheroidal, 
is not a perfectly solid case, but is perforated 
by very numerous foramina, both external and 
internal, by which large venous canals in the 
diploe of the bones communicate with the cir- 
culation of the integuments of the head as well 
as with that of the brain; so that the one can- 
not be materially affected without the other 
suffering likewise. And as the circulation in 
the integuments is not removed from atmo- 
pene pressure, neither can that which is so 
closely connected and continuous with it, be 
said to be free from the same influence. Still 
it must be admitted, that the deep position of 
the central vessels, and the complicated series 
of channels through which they communicate 
with the superficial ones, protect them in some 
‘degree from the pressure of the air, and render 
them less amenable to its influence than the 
vascular system of the surface. 
If it were essential to the integrity of the 
brain that the fluid in its bloodvessels should 
be protected from atmospheric pressure (as the 
advocates of this doctrine would have us to 
believe), a breach in the cranial wall would 
necessarily lead to the most injurious conse- 
quences ; yet, how frequently has the surgeon 
removed a large piece of the cranium by the 
trephine without any uatoward result ! me 
years ago I watched for several weeks a case in 
which nearly the whole of the upper part of 
the cranium had been removed by a process of 
necrosis, exposing a very large surface to the 
immediate pressure of the atmosphere ; yet in 
this case no disturbance of the cerebral circula- 
tion existed. In the large and open fonta- 
nelles of infants we have a state analogous to 
that which art or disease produces in the adult : 
NERVOUS SYSTEM. (Nervous Centres. Tus Encepnaton.) 
yet the vast majority of infants are free from 
cerebral disease for the whole period duri: 
which their crania remain incomplete ; and in 
infinitely the greatest number of cases in which 
children suffer from cerebral disease, the pri 
mary source of irritation is in some distan 
organ, and not in the brain itself. - 
It cannot be said that the brain is incom 
pressible. That only is incompressible, th 
particles of which will not admit of being moi 
closely packed together under the influence o 
pressure. That the brain is not a substance 
this kind is proved by the fact that, while it 
always undergoing a certain degree of pressui 
as essential to the integrity of its functions, 
slight increase of that pressure is sufficie 
produce such an amount of physical change ii 
it as at once to interfere with its healthy actior 
Too much blood distributed among its element 
and too much serum effused upon its surfa 
are equally capable of producing such an effeet 
Majendie’s experiments, described in a fo 
mer part of this article, show that the braii 
and spinal cord are surrounded by fluid, # 
pressure of which must an ise that 
is exerted through the bloodvessels. The 1 
a of this fluid i —_ ns ¢ 
ese centres, a ntly by allowing ass 
to become too fall. The preane exerted | 
the former may be called the fluid pressure fro 
without the brain; that by the blood, the pi 
sure from within. As long as these two a 
balanced, the brain enjoys a healthy state 
function, supposing its texture to be norn 
If either prevail, more or less of disturk 
will ensue. Their relative quantities, if not 
just proportion, will bear an inverse ratio’ 
each other. If there be much blood, the 
rounding fluid will be totally, or in a gr 
measure, deficient ; if the brain be aneen 
quantity of surrounding fluid will be large. 
The existence of these two antagonizing for 
may be taken as a proof that either of th 
may prevail ; and, therefore, from the pres 
of the cerebro-spinal fluid, we ae fe rs 
the actual quantity of circulating blood in” 
brain is liable to variation. 
The cerebro-spinal fluid is a valuable t 
lator of vascular fullness within the cranii 
and a protector of the brain against too m 
pressure from within. So long as it exis! 
normal quantity it resists the entrance of © 
than a certain proportion of blood in 
vessels. Under the influence of an un 
force of the heart an undue quantity of b 
may be forced into the brain, the effe 
which will be, first, the displacement of. 
or of the whole surrounding fluid; and, 
the compression of the brain. ~-_ 
When the brain receives too little blood 
requisite degree of pressure will be mainta 
and the healthy cerebral action preserved, | 
surrounding fluid do not increase too raf 
But if the brain be deprived of its due pi 
tion of blood by some sudden depression Of 
heart’s power, there is neither time nor Sot 
for the pouring out of a new fluid, anda 
of syncope or of delirium will ensue. * 
seems to be the explanation of those cas 
. ] ‘ 
ha 
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