In comparing the preceding system with this, it is difficult to
understand how they can exactly correspond, how the pulmonary can
transmit not only all that passes through the general, but also all the
lymph that returns from the serous surfaces and the cellular cavities,
all the chyle which enters by digestion, &c. &c.
It seems impossible at first view, that in the balance of the
circulation, these capillaries can, constantly and regularly, keep
in equilibrium with those of the rest of the body. By reflecting a
little, however, upon the phenomena of this function, we see that the
discordance is only apparent.
Though the general capillary system is everywhere spread out, yet the
portion in which blood circulates is much more limited than at first
appears. There is a great part of the vessels of this system, in
which fluids differing from the blood move and oscillate in different
directions. Then, where the blood especially enters, as in the muscles,
the mucous surfaces, &c. a considerable portion of this fluid, its
colouring matter particularly, is in a combined state, and not in a
state of circulation. If we cut a muscle transversely in a living
animal, inspection proves clearly this phenomenon, which, joined to the
preceding, diminishes immediately more than half the blood, which at
first appears to move in the general capillary system.
Yet it is evident, that there remains much more of it constantly in
this system than in the pulmonary; to be convinced of this, it is
only necessary to cut the lungs of a living animal. From this it is
clear, that if the heart presided over the motion of the blood in the
general system, and that consequently all that is contained in it was
driven into the veins at each pulsation, the pulmonary capillaries
would be insufficient to transmit it; but there goes out only a certain
quantity, proportioned to what the lungs can receive. It is nearly
the same as when the veins are much dilated, and consequently contain
much blood; no more arrives at the heart, because, as I have said, the
velocity is then in the inverse ratio of the capacity.
Besides, many causes continually divert the blood of the general
capillary system from the direction which carries it from the arteries
to the veins; these causes are especially the exhalations, secretions,
and nutrition. This capillary system is, as I have observed, a common
reservoir, whence the blood is carried into different and even opposite
directions, on one part in the direction of the veins, on another in
that of the exhalants, on another in that of the excretories, on
another, in fine, in that of the nutritive vessels. On the contrary,
in the pulmonary capillary system, there is but a single impulse, and
a single direction; it is that which carries the blood from the artery
to the pulmonary veins, which nothing draws off in its course; for in
passing from the black to the red, this fluid serves no other function;
it has no vessels, but the pulmonary veins, towards which its motion
is directed. There is, then, this great difference in the blood of the
pulmonary capillaries, and that of all the other parts, viz. that the
first is moved only in one direction, that all which arrives in the
lungs goes immediately in this direction; whereas the second has four
or five different directions. Hence this last necessarily oscillates
and varies in its motions, according as it is called more or less
powerfully by the exhalants, the excretories, the nourishing vessels,
or the veins; whereas the other, having but one way to escape, follows
it constantly and uniformly. Let us not be astonished, then, at the
disproportion there is in the capacity of the two capillary systems.
The proximity and distance of the heart are also a real cause that
tends to establish the equilibrium between the two systems. We have
seen, in fact, that each contraction of the left ventricle impresses a
sudden motion upon the whole mass of blood contained in the arteries,
and at the instant that this mass increases on one side, it is
diminished on the other by the quantity that is sent to the capillaries
of the whole body; so that the arterial motion is not progressive,
but sudden and instantaneous, so that at the same time the column of
aortic blood increases towards the heart, it diminishes in its remote
ramifications, and the fluid driven from the heart at each contraction,
does not arrive at the capillaries until after many contractions, since
that which goes from this organ cannot arrive at these vessels until
all which is before it has reached them. The same phenomenon precisely
takes place as it respects the black blood in the pulmonary artery.
Then the longer the course, the longer is the time that is required
for the blood to arrive at the capillaries, and consequently to pass
through them; then the blood from the right ventricle would arrive much
sooner at the left auricle, than that would at the right auricle which
goes from the left ventricle; then, though in what we call the small
circulation, the velocity is not greater, the space passed over being
less, the time employed to go over it is also less; then, the excess
of the fluid contained in the divisions of the aorta, in the general
capillary system, and in the general veins, over that contained in the
pulmonary artery, veins and capillary system, is compensated by the
time the second takes to go its course, which is short in comparison to
that of the first.
Hence we see, why in animals in whom the lungs, as to their
circulation, are in opposition to the rest of the body, nature has
constantly placed this organ at the side of the heart. If one of these
organs was at the head, and the other at the bottom of the pelvis, the
harmony would be inevitably interrupted.