6 O B S E R V A T I O N S ON THE
contains the fame quantity of blood, and, of courfe, that the arteries within
the head arc immoveable, Uke metalhne tubes, or want pulfation, or are
unfufccptible of innammation. For, whilft the heart is performing its fyftole,
the arteries here, as elfewhere, may be dilating, and, in the mean
time, a quantity of blood, equal to that which is dilating them, is paffing
B
out of the head by the veins.
During the fucceeding period of diaftole of the heart and fyftole of the
arteries, the quantity which dilated the arteries of the brain paffes into the
correfponding veins and fmufes ; at the fame rime, as much paffes from the
finufes out of the head, as enters into the head from the contraaing trunks
of the arteries iituated between the heart and the head.
T . VI.
NEITHER does it follow, from nearly the fame quantity of blood being at
all rimes contained within the head, that opening the arteries or veins on
the outfide of the head, or in the Hmbs, can be of no fervice in the cure of
inflammation, apoplexy, and other difeafes of the brain. For, although we
cannot, by arteriotomy or venefeaion, leffen much the quantity of the blood
within the cranium, we can diminiih the force with which it is impelled into it.
The great effefl: of this circumftance needs no other proof than that, in
cafes of faintncfs, brought on by inanition, we are reheved by the horizontal
poilure, and, in cafes of plethora, or of inflammation of the brain, we
are oppreffcd by that pofliure. Nay, the lefs compreffible we fuppofe the
fubllance of the brain to be, riie more readily we underftand how the whole
of it may be affeaed by a plethora, or increafed momentum of the blood ;
or a particular part of it injured by an inflammation, or by an extravafation
of the blood, or by any other caufe of preffure upon its fubftance.
T . VII.
IN phrenitis, apoplexy, and other difeafes, it has been propofed to perform
the operation of the trepan, in order that the external air admitted
might, by its preffure, leffen the diameter of the veffels.
But,
N E R V O U S S Y S T E M . ;
But, allowing we were to lay out of the quellion the dangerous effeds
which the air has upon all our deep feated organs, I would obferve, that,
if the diameter of the veffels fliould be diminiflied by the admiffion of the
air, this effea can only be explained on the principle, that the veffels are exposed
to more preffure after, than they were before trepanning, from which
furely no good effefts can be expeSed. We might, almofl:, as well propofe,
in peripneumony, to perform the paracentefis of the thorax.
T. VIII.
BEFORE difmiffmg this fubjea, it may be worth while to menrion, that,
in 1752, I made an experiment, with the affiftance of Dr Brodie, which
plainly flrewed, that, in animals killed by hanging, death was not, as Petit
fuppofed, owing chiefly to the preffure on the veffels of the brain, but depended
on the ftoppage of refpiration.
I cut a large hole in the trachea of a living dog. I then fufpended him
for three quarters of an hour, by a rope fixed around his neck above that
hole, without killing him or depriving him either of fenfe or motion. But,
when we afterwards fufpended him, for a quarter of an hour, by a rope
fixed under that hole, he became infenfible, and did not recover.
Dr Stolte, an ingenious Dutch phyfician, has confirmed this fad, by a
variety of fimilar experiments, of which he has given an account in his Inaugural
Differtation in 1764.
C H A P .