THE THIRD POSITION IS CONFIRMED: AND THE CIRCULATION OF THE BLOOD
IS DEMONSTRATED FROM IT
Thus far we have 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 cognisable 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 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 attempts 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 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 toward 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 portae. Let it be added that there are no
valves in the arteries, and that dogs, oxen, etc., 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.
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 the 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.
[NOTE.--Woodcuts of the veins of the arm to which these letters
and figures refer appear here in the original.--C. N. B. C]
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 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.