THE THIRD POSITION IS CONFIRMED: AND THE CIRCULATION OF THE BLOOD IS
DEMONSTRATED FROM IT.
Thus far have we spoken of the quantity of blood passing through the
heart and the lungs in the centre of the body, and in like manner from
the arteries into the veins in the peripheral parts and the body at
large. We have yet to explain, however, in what manner the blood finds
its way back to the heart from the extremities by the veins, and how and
in what way these are the only vessels that convey the blood from the
external to the central parts; which done, I conceive that the three
fundamental propositions laid down for the circulation of the blood will
be so plain, so well established, so obviously true, that they may claim
general credence. Now the remaining position will be made sufficiently
clear from the valves which are found in the cavities of the veins
themselves, from the uses of these, and from experiments cognizable by
the senses.
The celebrated Hieronymus Fabricius of Aquapendente, a most skilful
anatomist, and venerable old man, or, as the learned Riolan will have
it, Jacobus Silvius, first gave representations of the valves in the
veins, which consist of raised or loose portions of the inner membranes
of these vessels, of extreme delicacy, and a sigmoid or semilunar shape.
They are situated at different distances from one another, and
diversely in different individuals; they are connate at the sides of the
veins; they are directed upwards or towards the trunks of the veins; the
two--for there are for the most part two together--regard each other,
mutually touch, and are so ready to come into contact by their edges,
that if anything attempt to pass from the trunks into the branches of
the veins, or from the greater vessels into the less, they completely
prevent it; they are farther so arranged, that the horns of those that
succeed are opposite the middle of the convexity of those that precede,
and so on alternately.
The discoverer of these valves did not rightly understand their use, nor
have succeeding anatomists added anything to our knowledge: for their
office is by no means explained when we are told that it is to hinder
the blood, by its weight, from all flowing into inferior parts; for the
edges of the valves in the jugular veins hang downwards, and are so
contrived that they prevent the blood from rising upwards; the valves,
in a word, do not invariably look upwards, but always towards the trunks
of the veins, invariably towards the seat of the heart. I, and indeed
others, have sometimes found valves in the emulgent veins, and in those
of the mesentery, the edges of which were directed towards the vena cava
and vena portæ. Let it be added that there are no valves in the arteries
[save at their roots], and that dogs, oxen, &c., have invariably valves
at the divisions of their crural veins, in the veins that meet towards
the top of the os sacrum, and in those branches which come from the
haunches, in which no such effect of gravity from the erect position was
to be apprehended. Neither are there valves in the jugular veins for the
purpose of guarding against apoplexy, as some have said; because in
sleep the head is more apt to be influenced by the contents of the
carotid arteries. Neither are the valves present, in order that the
blood may be retained in the divarications or smaller trunks and minuter
branches, and not be suffered to flow entirely into the more open and
capacious channels; for they occur where there are no divarications;
although it must be owned that they are most frequent at the points
where branches join. Neither do they exist for the purpose of rendering
the current of blood more slow from the centre of the body; for it seems
likely that the blood would be disposed to flow with sufficient slowness
of its own accord, as it would have to pass from larger into
continually smaller vessels, being separated from the mass and fountain
head, and attaining from warmer into colder places.
But the valves are solely made and instituted lest the blood should pass
from the greater into the lesser veins, and either rupture them or cause
them to become varicose; lest, instead of advancing from the extreme to
the central parts of the body, the blood should rather proceed along the
veins from the centre to the extremities; but the delicate valves, while
they readily open in the right direction, entirely prevent all such
contrary motion, being so situated and arranged, that if anything
escapes, or is less perfectly obstructed by the cornua of the one above,
the fluid passing, as it were, by the chinks between the cornua, it is
immediately received on the convexity of the one beneath, which is
placed transversely with reference to the former, and so is effectually
hindered from getting any farther.
And this I have frequently experienced in my dissections of the veins:
if I attempted to pass a probe from the trunk of the veins into one of
the smaller branches, whatever care I took I found it impossible to
introduce it far any way, by reason of the valves; whilst, on the
contrary, it was most easy to push it along in the opposite direction,
from without inwards, or from the branches towards the trunks and roots.
In many places two valves are so placed and fitted, that when raised
they come exactly together in the middle of the vein, and are there
united by the contact of their margins; and so accurate is the
adaptation, that neither by the eye nor by any other means of
examination can the slightest chink along the line of contact be
perceived. But if the probe be now introduced from the extreme towards
the more central parts, the valves, like the floodgates of a river, give
way, and are most readily pushed aside. The effect of this arrangement
plainly is to prevent all motion of the blood from the heart and vena
cava, whether it be upwards towards the head, or downwards towards the
feet, or to either side towards the arms, not a drop can pass; all
motion of the blood, beginning in the larger and tending towards the
smaller veins, is opposed and resisted by them; whilst the motion that
proceeds from the lesser to end in the larger branches is favoured, or,
at all events, a free and open passage is left for it.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
[Illustration: Fig. 3.]
[Illustration: Fig. 4.]
But that this truth may be made the more apparent, let an arm be tied up
above the elbow as if for phlebotomy (A, A, fig. 1). At intervals in the
course of the veins, especially in labouring people and those whose
veins are large, certain knots or elevations (B, C, D, E, F,) will be
perceived, and this not only at the places where a branch is received
(E, F), but also where none enters (C, D): these knots or risings are
all formed by valves, which thus show themselves externally. And now if
you press the blood from the space above one of the valves, from H to O,
(fig. 2,) and keep the point of a finger upon the vein inferiorly, you
will see no influx of blood from above; the portion of the vein between
the point of the finger and the valve O will be obliterated; yet will
the vessel continue sufficiently distended above that valve (O, G). The
blood being thus pressed out, and the vein emptied, if you now apply a
finger of the other hand upon the distended part of the vein above the
valve O, (fig. 3,) and press downwards, you will find that you cannot
force the blood through or beyond the valve; but the greater effort you
use, you will only see the portion of vein that is between the finger
and the valve become more distended, that portion of the vein which is
below the valve remaining all the while empty (H, O, fig. 3).
It would therefore appear that the function of the valves in the veins
is the same as that of the three sigmoid valves which we find at the
commencement of the aorta and pulmonary artery, viz., to prevent all
reflux of the blood that is passing over them.
Farther, the arm being bound as before, and the veins looking full and
distended, if you press at one part in the course of a vein with the
point of a finger (L, fig. 4), and then with another finger streak the
blood upwards beyond the next valve (N), you will perceive that this
portion of the vein continues empty (L N), and that the blood cannot
retrograde, precisely as we have already seen the case to be in fig. 2;
but the finger first applied (H, fig. 2, L, fig. 4), being removed,
immediately the vein is filled from below, and the arm becomes as it
appears at D C, fig. 1. That the blood in the veins therefore proceeds
from inferior or more remote to superior parts, and towards the heart,
moving in these vessels in this and not in the contrary direction,
appears most obviously. And although in some places the valves, by not
acting with such perfect accuracy, or where there is but a single valve,
do not seem totally to prevent the passage of the blood from the centre,
still the greater number of them plainly do so; and then, where things
appear contrived more negligently, this is compensated either by the
more frequent occurrence or more perfect action of the succeeding valves
or in some other way: the veins, in short, as they are the free and open
conduits of the blood returning _to_ the heart, so are they effectually
prevented from serving as its channels of distribution _from_ the heart.
But this other circumstance has to be noted: The arm being bound, and
the veins made turgid, and the valves prominent, as before, apply the
thumb or finger over a vein in the situation of one of the valves in
such a way as to compress it, and prevent any blood from passing upwards
from the hand; then, with a finger of the other hand, streak the blood
in the vein upwards till it has passed the next valve above, (N, fig.
4,) the vessel now remains empty; but the finger at L being removed for
an instant, the vein is immediately filled from below; apply the finger
again, and having in the same manner streaked the blood upwards, again
remove the finger below, and again the vessel becomes distended as
before; and this repeat, say a thousand times, in a short space of time.
And now compute the quantity of blood which you have thus pressed up
beyond the valve, and then multiplying the assumed quantity by one
thousand, you will find that so much blood has passed through a certain
portion of the vessel; and I do now believe that you will find yourself
convinced of the circulation of the blood, and of its rapid motion. But
if in this experiment you say that a violence is done to nature, I do
not doubt but that, if you proceed in the same way, only taking as great
a length of vein as possible, and merely remark with what rapidity the
blood flows upwards, and fills the vessel from below, you will come to
the same conclusion.