OF THE COURSE BY WHICH THE BLOOD IS CARRIED FROM THE VENA CAVA INTO THE
ARTERIES, OR FROM THE RIGHT INTO THE LEFT VENTRICLE OF THE HEART.
Since the intimate connexion of the heart with the lungs, which is
apparent in the human subject, has been the probable cause of the errors
that have been committed on this point, they plainly do amiss who,
pretending to speak of the parts of animals generally, as anatomists for
the most part do, confine their researches to the human body alone, and
that when it is dead. They obviously act no otherwise than he who,
having studied the forms of a single commonwealth, should set about the
composition of a general system of polity; or who, having taken
cognizance of the nature of a single field, should imagine that he had
mastered the science of agriculture; or who, upon the ground of one
particular proposition, should proceed to draw general conclusions.
Had anatomists only been as conversant with the dissection of the lower
animals as they are with that of the human body, the matters that have
hitherto kept them in a perplexity of doubt would, in my opinion, have
met them freed from every kind of difficulty.
And, first, in fishes, in which the heart consists of but a single
ventricle, they having no lungs, the thing is sufficiently manifest.
Here the sac, which is situated at the base of the heart, and is the
part analogous to the auricle in man, plainly throws the blood into the
heart, and the heart, in its turn, conspicuously transmits it by a pipe
or artery, or vessel analogous to an artery; these are facts which are
confirmed by simple ocular inspection, as well as by a division of the
vessel, when the blood is seen to be projected by each pulsation of the
heart.
The same thing is also not difficult of demonstration in those animals
that have either no more, or, as it were, no more than a single
ventricle to the heart, such as toads, frogs, serpents, and lizards,
which, although they have lungs in a certain sense, as they have a
voice, (and I have many observations by me on the admirable structure of
the lungs of these animals, and matters appertaining, which, however, I
cannot introduce in this place,) still their anatomy plainly shows that
the blood is transferred in them from the veins to the arteries in the
same manner as in higher animals, viz., by the action of the heart; the
way, in fact, is patent, open, manifest; there is no difficulty, no room
for hesitating about it; for in them the matter stands precisely as it
would in man, were the septum of his heart perforated or removed, or one
ventricle made out of two; and this being the case, I imagine that no
one will doubt as to the way by which the blood may pass from the veins
into the arteries.
But as there are actually more animals which have no lungs than there
are which be furnished with them, and in like manner a greater number
which have only one ventricle than there are which have two, it is open
to us to conclude, judging from the mass or multitude of living
creatures, that for the major part, and generally, there is an open way
by which the blood is transmitted from the veins through the sinuses or
cavities of the heart into the arteries.
I have, however, cogitating with myself, seen further, that the same
thing obtained most obviously in the embryos of those animals that have
lungs; for in the fœtus the four vessels belonging to the heart, viz.,
the vena cava, the vena arteriosa or pulmonary artery, the arteria
venalis or pulmonary vein, and the arteria magna or aorta, are all
connected otherwise than in the adult; a fact sufficiently known to
every anatomist. The first contact and union of the vena cava with the
arteria venosa or pulmonary veins, which occurs before the cava opens
properly into the right ventricle of the heart, or gives off the
coronary vein, a little above its escape from the liver, is by a lateral
anastomosis; this is an ample foramen, of an oval form, communicating
between the cava and the arteria venosa, or pulmonary vein, so that the
blood is free to flow in the greatest abundance by that foramen from the
vena cava into the arteria venosa or pulmonary vein, and left auricle,
and from thence into the left ventricle; and farther, in this foramen
ovale, from that part which regards the arteria venosa, or pulmonary
vein, there is a thin tough membrane, larger than the opening, extended
like an operculum or cover; this membrane in the adult blocking up the
foramen, and adhering on all sides, finally closes it up, and almost
obliterates every trace of it. This membrane, however, is so contrived
in the fœtus, that falling loosely upon itself, it permits a ready
access to the lungs and heart, yielding a passage to the blood which is
streaming from the cava, and hindering the tide at the same time from
flowing back into that vein. All things, in short, permit us to believe
that in the embryo the blood must constantly pass by this foramen from
the vena cava into the arteria venosa, or pulmonary vein, and from
thence into the left auricle of the heart; and having once entered
there, it can never regurgitate.
Another union is that by the vena arteriosa, or pulmonary artery, and is
effected when that vessel divides into two branches after its escape
from the right ventricle of the heart. It is as if to the two trunks
already mentioned a third were superadded, a kind of arterial canal,
carried obliquely from the vena arteriosa, or pulmonary artery, to
perforate and terminate in the arteria magna or aorta. In the embryo,
consequently, there are, as it were, two aortas, or two roots of the
arteria magna, springing from the heart. This canalis arteriosus shrinks
gradually after birth, and is at length and finally almost entirely
withered, and removed, like the umbilical vessels.
The canalis arteriosus contains no membrane or valve to direct or impede
the flow of the blood in this or in that direction: for at the root of
the vena arteriosa, or pulmonary artery, of which the canalis arteriosus
is the continuation in the fœtus, there are three sigmoid or semilunar
valves, which open from within outwards, and oppose no obstacle to the
blood flowing in this direction or from the right ventricle into the
pulmonary artery and aorta; but they prevent all regurgitation from the
aorta or pulmonic vessels back upon the right ventricle; closing with
perfect accuracy, they oppose an effectual obstacle to everything of the
kind in the embryo. So that there is also reason to believe that when
the heart contracts, the blood is regularly propelled by the canal or
passage indicated from the right ventricle into the aorta.
What is commonly said in regard to these two great communications, to
wit, that they exist for the nutrition of the lungs, is both improbable
and inconsistent; seeing that in the adult they are closed up,
abolished, and consolidated, although the lungs, by reason of their heat
and motion, must then be presumed to require a larger supply of
nourishment. The same may be said in regard to the assertion that the
heart in the embryo does not pulsate, that it neither acts nor moves, so
that nature was forced to make these communications for the nutrition of
the lungs. This is plainly false; for simple inspection of the incubated
egg, and of embryos just taken out of the uterus, shows that the heart
moves precisely in them as in adults, and that nature feels no such
necessity. I have myself repeatedly seen these motions, and Aristotle is
likewise witness of their reality. “The pulse,” he observes, “inheres in
the very constitution of the heart, and appears from the beginning, as
is learned both from the dissection of living animals, and the formation
of the chick in the egg.”[91] But we further observe, that the passages
in question are not only pervious up to the period of birth in man, as
well as in other animals, as anatomists in general have described them,
but for several months subsequently, in some indeed for several years,
not to say for the whole course of life; as, for example, in the goose,
snipe, and various birds, and many of the smaller animals. And this
circumstance it was, perhaps, that imposed upon Botallus, who thought he
had discovered a new passage for the blood from the vena cava into the
left ventricle of the heart; and I own that when I met with the same
arrangement in one of the larger members of the mouse family, in the
adult state, I was myself at first led to something of a like
conclusion.
From this it will be understood that in the human embryo, and in the
embryos of animals in which the communications are not closed, the same
thing happens, namely, that the heart by its motion propels the blood by
obvious and open passages from the vena cava into the aorta through the
cavities of both the ventricles; the right one receiving the blood from
the auricle, and propelling it by the vena arteriosa, or pulmonary
artery, and its continuation, named the ductus arteriosus, into the
aorta; the left, in like manner, charged by the contraction of its
auricle, which has received its supply through the foramen ovale from
the vena cava, contracting, and projecting the blood through the root of
the aorta into the trunk of that vessel.
In embryos, consequently, whilst the lungs are yet in a state of
inaction, performing no function, subject to no motion any more than if
they had not been present, nature uses the two ventricles of the heart
as if they formed but one, for the transmission of the blood. The
condition of the embryos of those animals which have lungs, whilst these
organs are yet in abeyance and not employed, is the same as that of
those animals which have no lungs.
So clearly, therefore, does it appear in the case of the fœtus, viz.,
that the heart by its action transfers the blood from the vena cava into
the aorta, and that by a route as obvious and open, as if in the adult
the two ventricles were made to communicate by the removal of their
septum. Since, then, we find that in the greater number of animals, in
all, indeed, at a certain period of their existence, the channels for
the transmission of the blood through the heart are so conspicuous, we
have still to inquire wherefore in some creatures--those, namely, that
have warm blood, and that have attained to the adult age, man among the
number--we should not conclude that the same thing is accomplished
through the substance of the lungs, which in the embryo, and at a time
when the function of these organs is in abeyance, nature effects by the
direct passages described, and which, indeed, she seems compelled to
adopt through want of a passage by the lungs; or wherefore it should be
better (for nature always does that which is best) that she should close
up the various open routes which she had formerly made use of in the
embryo and fœtus, and still uses in all other animals; not only opening
up no new apparent channels for the passage of the blood, therefore, but
even entirely shutting up those which formerly existed.
And now the discussion is brought to this point, that they who inquire
into the ways by which the blood reaches the left ventricle of the heart
and pulmonary veins from the vena cava, will pursue the wisest course if
they seek by dissection to discover the causes why in the larger and
more perfect animals of mature age, nature has rather chosen to make the
blood percolate the parenchyma of the lungs, than as in other instances
chosen a direct and obvious course--for I assume that no other path or
mode of transit can be entertained. It must be either because the larger
and more perfect animals are warmer, and when adult their heat
greater--ignited, as I might say, and requiring to be damped or
mitigated; therefore it may be that the blood is sent through the
lungs, that it may be tempered by the air that is inspired, and
prevented from boiling up, and so becoming extinguished, or something
else of the sort. But to determine these matters, and explain them
satisfactorily, were to enter on a speculation in regard to the office
of the lungs and the ends for which they exist; and upon such a subject,
as well as upon what pertains to eventilation, to the necessity and use
of the air, &c., as also to the variety and diversity of organs that
exist in the bodies of animals in connexion with these matters, although
I have made a vast number of observations, still, lest I should be held
as wandering too wide of my present purpose, which is the use and motion
of the heart, and be charged with speaking of things beside the
question, and rather complicating and quitting than illustrating it, I
shall leave such topics till I can more conveniently set them forth in a
treatise apart. And now, returning to my immediate subject, I go on with
what yet remains for demonstration, viz., that in the more perfect and
warmer adult animals, and man, the blood passes from the right ventricle
of the heart by the vena arteriosa, or pulmonary artery, into the lungs,
and thence by the arteriæ venosæ, or pulmonary veins, into the left
auricle, and thence into the left ventricle of the heart. And, first, I
shall show that this may be so, and then I shall prove that it is so in
fact.