THE BLOOD PASSES THROUGH THE SUBSTANCE OF THE LUNGS FROM THE
RIGHT VENTRICLE OF THE HEART INTO THE PULMONARY VEINS AND LEFT
VENTRICLE
That this is possible, and that there is nothing to prevent it
from being so, appears when we reflect on the way in which water
permeating the earth produces springs and rivulets, or when we
speculate on the means by which the sweat passes through the
skin, or the urine through the substance of the kidneys. It is
well known that persons who use the Spa waters or those of La
Madonna, in the territories of Padua, or others of an acidulous
or vitriolated nature, or who simply swallow drinks by the
gallon, pass all off again within an hour or two by the bladder.
Such a quantity of liquid must take some short time in the
concoction: it must pass through the liver (it is allowed by all
that the juices of the food we consume pass twice through this
organ in the course of the day); it must flow through the veins,
through the tissues of the kidneys, and through the ureters into
the bladder.
To those, therefore, whom I hear denying that the blood, aye, the
whole mass of the blood, may pass through the substance of the
lungs, even as the nutritive juices percolate the liver,
asserting such a proposition to be impossible, and by no means to
be entertained as credible, I reply, with the poet, that they are
of that race of men who, when they will, assent full readily, and
when they will not, by no manner of means; who, when their assent
is wanted, fear, and when it is not, fear not to give it.
The substance of the liver is extremely dense, so is that of the
kidney; the lungs, however, are of a much looser texture, and if
compared with the kidneys are absolutely spongy. In the liver
there is no forcing, no impelling power; in the lungs the blood
is forced on by the pulse of the right ventricle, the necessary
effect of whose impulse is the distension of the vessels and the
pores of the lungs. And then the lungs, in respiration, are
perpetually rising and falling: motions, the effect of which must
needs be to open and shut the pores and vessels, precisely as in
the case of a sponge, and of parts having a spongy structure,
when they are alternately compressed and again are suffered to
expand. The liver, on the contrary, remains at rest, and is never
seen to be dilated or constricted. Lastly, if no one denies the
possibility in man, oxen, and the larger animals generally, of
the whole of the ingested juices passing through the liver, in
order to reach the vena cava, for this reason, that if
nourishment is to go on, these juices must needs get into the
veins, and there is no other way but the one indicated, why
should not the same arguments be held of avail for the passage of
the blood in adults through the lungs? Why not maintain, with
Columbus, that skilful and learned anatomist, that it must be so
from the capacity and structure of the pulmonary vessels, and
from the fact of the pulmonary veins and ventricle corresponding
with them, being always found to contain blood, which must needs
have come from the veins, and by no other passage save through
the lungs? Columbus, and we also, from what precedes, from
dissections, and other arguments, conceive the thing to be clear.
But as there are some who admit nothing unless upon authority,
let them learn that the truth I am contending for can be
confirmed from Galen's own words, namely, that not only may the
blood be transmitted from the pulmonary artery into the pulmonary
veins, then into the left ventricle of the heart, and from thence
into the arteries of the body, but that this is effected by the
ceaseless pulsation of the heart and the motion of the lungs in
breathing.
There are, as everyone knows, three sigmoid or semilunar valves
situated at the orifice of the pulmonary artery, which
effectually prevent the blood sent into the vessel from returning
into the cavity of the heart. Now Galen, explaining the use of
these valves, and the necessity for them, employs the following
language: [Footnote: De Usu partium, lib. vi, cap. 10] "There is
everywhere a mutual anastomosis and inosculation of the arteries
with the veins, and they severally transmit both blood and
spirit, by certain invisible and undoubtedly very narrow
passages. Now if the mouth of the pulmonary artery had stood in
like manner continually open, and nature had found no contrivance
for closing it when requisite, and opening it again, it would
have been impossible that the blood could ever have passed by the
invisible and delicate mouths, during the contractions of the
thorax, into the arteries; for all things are not alike readily
attracted or repelled; but that which is light is more readily
drawn in, the instrument being dilated, and forced out again when
it is contracted, than that which is heavy; and in like manner is
anything drawn more rapidly along an ample conduit, and again
driven forth, than it is through a narrow tube. But when the
thorax is contracted the pulmonary veins, which are in the lungs,
being driven inwardly, and powerfully compressed on every side,
immediately force out some of the spirit they contain, and at the
same time assume a certain portion of blood by those subtle
mouths, a thing that could never come to pass were the blood at
liberty to flow back into the heart through the great orifice of
the pulmonary artery. But its return through this great opening
being prevented, when it is compressed on every side, a certain
portion of it distils into the pulmonary veins by the minute
orifices mentioned." And shortly afterwards, in the next chapter,
he says: "The more the thorax contracts, the more it strives to
force out the blood, the more exactly do these membranes (viz.,
the semilunar valves) close up the mouth of the vessel, and
suffer nothing to regurgitate." The same fact he has also alluded
to in a preceding part of the tenth chapter: "Were there no
valves, a three-fold inconvenience would result, so that the
blood would then perform this lengthened course in vain; it would
flow inwards during the disastoles of the lungs and fill all
their arteries; but in the systoles, in the manner of the tide,
it would ever and anon, like the Euripus, flow backwards and
forwards by the same way, with a reciprocating motion, which
would nowise suit the blood. This, however, may seem a matter of
little moment: but if it meantime appear that the function of
respiration suffer, then I think it would be looked upon as no
trifle, etc." Shortly afterwards he says: "And then a third
inconvenience, by no means to be thought lightly of, would
follow, were the blood moved backwards during the expirations,
had not our Maker instituted those supplementary membranes. "In
the eleventh chapter he concludes: "That they (the valves) have
all a common use, and that it is to prevent regurgitation or
backward motion; each, however, having a proper function, the one
set drawing matters from the heart, and preventing their return,
the other drawing matters into the heart, and preventing their
escape from it. For nature never intended to distress the heart
with needless labour, neither to bring aught into the organ which
it had been better to have kept away, nor to take from it again
aught which it was requisite should be brought. Since, then,
there are four orifices in all, two in either ventricle, one of
these induces, the other educes." And again he says: "Farther,
since there is one vessel, which consists of a simple covering
implanted in the heart, and another which is double, extending
from it (Galen is here speaking of the right side of the heart,
but I extend his observations to the left side also), a kind of
reservoir had to be provided, to which both belonging, the blood
should be drawn in by one, and sent out by the other."
Galen adduces this argument for the transit of the blood by the
right ventricle from the vena cava into the lungs; but we can use
it with still greater propriety, merely changing the terms, for
the passage of the blood from the veins through the heart into
the arteries. From Galen, however, that great man, that father of
physicians, it clearly appears that the blood passes through the
lungs from the pulmonary artery into the minute branches of the
pulmonary veins, urged to this both by the pulses of the heart
and by the motions of the lungs and thorax; that the heart,
moreover, is incessantly receiving and expelling the blood by and
from its ventricles, as from a magazine or cistern, and for this
end it is furnished with four sets of valves, two serving for the
induction and two for the eduction of the blood, lest, like the
Euripus, it should be incommodiously sent hither and thither, or
flow back into the cavity which it should have quitted, or quit
the part where its presence was required, and so the heart might
be oppressed with labour in vain, and the office of the lungs be
interfered with. [Footnote: See the Commentary of the learned
Hofmann upon the Sixth Book of Galen, "De Usu partium," a work
which I first saw after I had written what precedes.] Finally,
our position that the blood is continually permeating from the
right to the left ventricle, from the vena cava into the aorta,
through the porosities of the lungs, plainly appears from this,
that since the blood is incessantly sent from the right ventricle
into the lungs by the pulmonary artery, and in like manner is
incessantly drawn from the lungs into the left ventricle, as
appears from what precedes and the position of the valves, it
cannot do otherwise than pass through continuously. And then, as
the blood is incessantly flowing into the right ventricle of the
heart, and is continually passed out from the left, as appears in
like manner, and as is obvious, both to sense and reason, it is
impossible that the blood can do otherwise than pass continually
from the vena cava into the aorta.
Dissection consequently shows distinctly what takes place in the
majority of animals, and indeed in all, up to the period of their
maturity; and that the same thing occurs in adults is equally
certain, both from Galen's words, and what has already been said,
only that in the former the transit is effected by open and
obvious passages, in the latter by the hidden porosities of the
lungs and the minute inosculations of vessels. It therefore
appears that, although one ventricle of the heart, the left to
wit, would suffice for the distribution of the blood over the
body, and its eduction from the vena cava, as indeed is done in
those creatures that have no lungs, nature, nevertheless, when
she ordained that the same blood should also percolate the lungs,
saw herself obliged to add the right ventricle, the pulse of
which should force the blood from the vena cava through the lungs
into the cavity of the left ventricle. In this way, it may be
said, that the right ventricle is made for the sake of the lungs,
and for the transmission of the blood through them, not for their
nutrition; for it were unreasonable to suppose that the lungs
should require so much more copious a supply of nutriment, and
that of so much purer and more spirituous a nature as coming
immediately from the ventricle of the heart, that either the
brain, with its peculiarly pure substance, or the eyes, with
their lustrous and truly admirable structure, or the flesh of the
heart itself, which is more suitably nourished by the coronary
artery.