O B S E R V A I M O N S ON THE
T. I I I .
SOME eminent phyfiologifts have regarded the finufes as confiderable reccptaclcs
of blood, and have propofed theories to explain the caufe or effedts
of the accumulation of blood in them. But, when we confider that
the vertebral arteries are accompanied by fmall veins only, and, of courfe,
that almoil the whole blood which thefe as well as the internal carotid arteries
convey, is returned by the lateral finufes and their continuation, the
internal jugular veins, it will appear, that the venous trunks within the
head are not much, if at all, larger than veins are, with refpeft to correfponding
arteries, in other bowels, as in the kidneys, fplcen, &c.
I V .
THE primary ufes of the ilrudlure defcribed are, therefore, i. That
when, by a fudden and violent adlion of the mufcles of expiration, or of
the mufcles of the head and neck, the blood is repelled in the cavae or internal
jugular veins, the impulfe may not be communicated to the blood in
the fmall and tender veins of the brain, which would endanger a rupture
of thefe. To confii-m which, it may be obferved, that, generally, a very
fmall quantity only of any liquor injeded into the finufes enters the fmall
veins of the brain and cerebellum.
2. The dura mater, invefting the trunks of the veins of the brain and
•^cerebellum, evidently ilrengthens thefe, and prevents their being eafily
ruptured.
3. The dura mater ftretched over the finufes, which are lodged in furrows
of bones, and its procelTes, the falx and tentorium, connefted to
angles of »^he finufes, prevent the oppofite fides of the finufes from being
prefled nearer to each.other, or prevent their cavities from being much diminilhed
by the weight of the parts of the brain and cerebellum in different
N E R V O U S S Y S T E M ;
rent poftures; or by an increafed impulfe of the blood in the arteries; or
by water accumulated in the ventricles of the brain, or on its furface; or by
fcirrhous or other tumours forming in its fubftance.
To illuilrate this efted of the falx and tentorium upon the finufes: If,
after taking out the brain and cerebellum, we cut the falx and tentorium, *
and dry the head, the cavities of the fuperior longitudinal finus, of the two
lateral finufes, and of the torcular Herophili, will be found much diminiihed:
But, if the falx and tentorium are preferved entire, though we dry • the
head, the cavities of the finufes will be fo httle diminillied, that we would
fuppofe they had been purpofely kept inflated, whilil the head was drying.
4. The ends of the lateral finufes perforate the cranium at fome diftance
from the arteries, that the returning blood might, as little as poffible, fuffer
interruption in its courfe towards the heart.
For any alternate ftop of it, efpecially when occafioned by the ftroke of
the correfponding artery, muil have had a worfe effeit on the brain than on
other oi'gans ; not merely on account of the delicacy of the brain, and the
thinnefs of its veins, but becaufe, being inclofed in a cafe of bone, the blood
muft be continually flowing out by the veins, that room may be given to
the blood which is entering by the arteries. For, as the fubftance of the
brain, like that of the other folids of our body, is nearly incompreffible,
the quantity of blood within the head muft be the fame, or very nearly the
fame, at all times, whether in health or difeafe, in life or after death, thofe
cafes only excepted, in which water or other matter is effufed or fecreted
from the blood-velTels; for in thefe, a quantity of blood, equal in bulk to the
effufed matter, will be preifed out of the cranium.
E T. V .
IT does not, however, follow from this, that every individual artery or
vein within the head is conftantly of the fame fize, or that, at all times, it
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