THE BLOOD PERCOLATES 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 percolating
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 parenchyma 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 urine.
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 parenchyma 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 parenchyma of the liver is extremely dense, so is that of the
kidney; the lungs, again, 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 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 and constricted. Lastly, if no one denies the
possibility of the whole of the ingested juices passing through the
liver, in man, oxen, and the larger animals generally, in order to reach
the vena cava, and 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,
with Columbus, that skilful and learned anatomist, maintain and believe
the like, from the capacity and structure of the pulmonary vessels; 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 every one 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 uses of these valves, and the
necessity for them, employs the following language:[92] “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
vena arteriosa, or 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 subtile 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 very 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 sigmoid
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 diastoles 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, &c.” And
again, and shortly afterwards: “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 [the sigmoid valves].” Whence, in the eleventh
chapter, he concludes: “ That they have all a common use, (to wit, the
valves,) 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, consisting of a simple tunic, implanted in
the heart, and another, having a double tunic, 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 the
one, and sent out by the other.”
This argument Galen adduces 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 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 be oppressed with labour in
vain, and the office of the lungs be interfered with.[93] Finally, our
position that the blood is continually passing from the right to the
left ventricle, from the vena cava into the aorta, through the porous
structure 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 regard to
the transit of the blood] in the greater number of animals, and indeed
in all, up to the period of their [fœtal] maturity; and that the same
thing occurs in adults is equally certain, both from Galen’s words, and
what has already been said on the subject, only that in the former the
transit is effected by open and obvious passages, in the latter by the
obscure porosities of the lungs and the minute inosculations of
vessels. Whence it 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 another ventricle, the right, 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, therefore, 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; seeing
it were unreasonable to suppose that the lungs required any so much more
copious a supply of nutriment, and that of so much purer and more
spirituous a kind, as coming immediately from the ventricle of the
heart, than 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 commodiously nourished by the coronary
artery.