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 was 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.”[27] 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.
[27] Lib. de Spiritu, cap. v.
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.