O B S E R V A T I O N S ON THE
C H A P T E R
Of the Cii-culation of the Blood within the Head.
S E C T . I.
IT has been juftly obferved by authors, that the force of the blood fent
to the brain and cerebellum in man is broken by its afcent; by the
angles at the rife of the internal carotid and vertebral arteries ; by the turns
which thefe form in tlieir courfe, for inftance, by the turns of the internal
carotids within the os petrofum and at the iides of the fella turcica; and
by the uncommonly great proportion which tlie fum of the areae of the
branches bears to the area of the trunk.
But this intention of nature appears more evidently in the ruiTiinating
quadrupeds; for I find that a fubftance connected with the internal carotid
artery, obferved by Galen and named by him rete mirabile, which by
Heiiler and others has been called plexus vaforum et fibrarum, ufus incogniti*,
confifts entirely of a divifion of that artery into fmall ferpentine
branches, which are aftei-wards colleded, at the fide of the fella turcica, into
a trunk that is divided nearly as in man f ; and the vertebral arteries, I
likcwife obferve, form a rete mirabile.
But, although the momentum of the blood is thus broken, the quantity
of it which circulates in the brain is greater than in moft organs of the
fame weight. Thus one of our arms will be found to weigh more than
our brain and cerebellum; yet the areae of the two vertebral and of the
two internal carotid arteries, joined together, are much larger than the
area of the proper fubclavian artery.
This
• SecHuiller Comp. Anat. p. 138. 9 273. t See T. I.
N E R V O U S S Y S T E M .
This quantity of blood does not, however, fo greatly exceed the proportion
of that circulated in moil other parts, as Dr Keil, Dr Boerhaavej
and Dr Haller, have endeavoured to prove ; for thofe very learned authors,
inftead of comparing the areae of the internal carotid and vertebral arteries
with the area of the trunk of the defcending aorta ought to have compared
them with fimilar branches fent off from that tmnk. On a juft
comparifon, not above one tenth part of the whole mafs is circulated within
the head; which is, however, nearly four times more than, in general,
is circulated in the reft of the aortic fyftem, as the weight of the encephalon
does not exceed one fortieth part of the weight of the whole body.
T . II.
IN the brain, the fmall circulating veins are known to terminate in finufes,
or in trunks covered by the dura mater; and their paffage through
the dura mater, or between it and the proper trunk of the veins, is oblique
; nay, the mouths of many of thofe veins are turned in a direclion
contrary to the current of the blood in the finus f . But it is further to be
obferved, that, while one fide of the fmus is firmly fixed to the bone excavated
for its reception, the other fide of it eitlier has the dura mater
drawn tenfe over it, as in the finufes at the bafis of the cranium; or is drawn
out into an angle by the falx cerebri, or tentorium cerebello fuperextenfum,
as is the cafe of the fuperior longitudinal and part of the lateral finufes
; 01-, as in the torcular Herophili, the tentorium forms the bafis of the
triangular finus, whiift the oppofite angle is fupported by the falx. In the
lail place, the terminations of the great veins or finufes, or ends of the
lateral finufes, are not contiguous to the trunks of the correfponding arteries,
as is obferved in moft other parts of the body, but pafs through different
holes of the cranium.
S E C T ,
• Boerhaave Prael. in Inft. § 339. and Haller, El. Phyf, V. 4. p. 1
t Haller, El. Phyf. T. 4. p. 148.