Harvey CHAPTER
XI THE
SECOND POSITION IS DEMONSTRATED That this may the more
clearly appear to every one, I have here to cite certain experiments, from
which it seems obvious that the blood enters a limb by the arteries, and
returns from it by the veins; that the arteries are the vessels carrying
the blood from the heart, and the veins the returning channels of the
blood to the heart; that in the limbs and extreme parts of the body the
blood passes either immediately by anastomosis from the arteries into the
veins, or mediately by the porosities of the flesh, or in both ways, as
has already been said in speaking of the passage of the blood through the
lungs whence it appears manifest that in the circuit the blood moves from
that place to this place, and from that point to this one; from the centre
to the extremities, to wit; and from the extreme parts back again to the
centre. Finally, upon grounds of calculation, with the same elements as
before, it will be obvious that the quantity can neither be accounted for
by the ingesta, nor yet be held necessary to nutrition. The
same thing will also appear in regard to ligatures, and wherefore they are
said to draw; though this is
neither from the heat, nor the pain, nor the vacuum they occasion, nor
indeed from any other cause yet thought of; it will also explain the uses
and advantages to be derived from ligatures in medicine, the principle
upon which they either suppress or occasion hemorrhage; how they induce
sloughing and more extensive mortification in extremities; and how they
act in the castration of animals and the removal of warts and fleshy
tumours. But it has come to pass, from no one having duly weighed and
understood the causes, and rationale of these various effects, that though
almost all, upon the faith of the old writers, recommend ligatures in the
treatment of disease, yet very few comprehend their proper employment, or
derive any real assistance from them in effecting cures. Ligatures
are either very tight or of medium tightness. A ligature I designate as
tight or perfect when it so constricts an extremity that no vessel can be
felt pulsating beyond it. Such a ligature we use in amputations to control
the flow of blood; and such also are employed in the castration of animals
and the ablation of tumours. In the latter instances, all afflux of
nutriment and heat being prevented by the ligature, we see the testes and
large fleshy tumours dwindle, die, and finally fall off. Ligatures
of medium tightness I regard as those which compress a limb firmly all
round, but short of pain, and in such a way as still suffers a certain
degree of pulsation to be felt in the artery beyond them. Such a ligature
is in use in blood-letting, an operation which the fillet applied above
the elbow is not drawn so tight but that the arteries at the wrist may
still be felt beating under the finger. Now
let anyone make an experiment upon the arm of a man, either using such a
fillet as is employed in bloodletting, or grasping the limb lightly with
his hand, the best subject for it being one who is lean, and who has large
veins, and the best time after exercise, when the body is warm, the pulse
is full, and the blood carried in larger quantity to the extremities, for
all then is more conspicuous; under such circumstances let a ligature be
thrown about the extremity, and drawn as tightly as can be borne, it will
first be perceived that beyond the ligature, neither in the wrist nor
anywhere else, do the arteries pulsate, at the same time that immediately
above the ligature the artery begins to rise higher at each diastole, to
throb more violently, and to swell in its vicinity with a kind of tide, as
if it strove to break through and overcome the obstacle to its current;
the artery here, in short, appears as if it were preternaturally full. The
hand under such circumstances retains its natural colour and appearance;
in the course of time it begins to fall somewhat in temperature, indeed,
but nothing is drawn into it. After
the bandage has been kept on for some short time in this way, let it be
slackened a little, brought to that state or term of medium tightness
which is used in bleeding, and it will be seen that the whole hand and arm
will instantly become deeply coloured and distended, and the veins show
themselves tumid and knotted; after ten or twelve pulses of the artery,
the hand will be perceived excessively distended, injected, gorged with
blood, drawn, as it is said, by this medium ligature, without pain, or
heat, or any horror of a vacuum, or any other cause yet indicated. If
the finger be applied over the artery as it is pulsating by the edge of
the fillet, at the moment of slackening it, the blood will be felt to
glide through, as it were, underneath the finger; and he, too, upon whose
arm the experiment is made, when the ligature is slackened, is distinctly
conscious of a sensation of warmth, and of something, viz., a stream of
blood suddenly making its way along the course of the vessels and
diffusing itself through the hand, which at the same time begins to feel
hot, and becomes distended. As
we had noted, in connection with the tight ligature, that the artery above
the bandage was distended and pulsated, not below it, so, in the case of
the moderately tight bandage, on the contrary, do we find that the veins
below, never above, the fillet, swell, and become dilated, whilst the
arteries shrink; and such is the degree of distension of the veins here,
that it is only very strong pressure that will force the blood beyond the
fillet, and cause any of the veins in the upper part of the arm to rise. From
these facts it is easy for every careful observer to learn that the blood
enters an extremity by the arteries; for when they are effectually
compressed nothing is drawn to the member; the hand preserves its colour;
nothing flows into it, neither is it distended; but when the pressure is
diminished, as it is with the bleeding fillet, it is manifest that the
blood is instantly thrown in with force, for then the hand begins to
swell; which is as much as to say, that when the arteries pulsate the
blood is flowing through them, as it is when the moderately tight ligature
is applied; but where they do not pulsate, as, when a tight ligature is
used, they cease from transmitting anything, they are only distended above
the part where the ligature is applied. The veins again being compressed,
nothing can flow through them; the certain indication of which is, that
below the ligature they are much more tumid than above it, and than they
usually appear when there is no bandage upon the arm. It
therefore plainly appears that the ligature prevents the return of the
blood through the veins to the parts above it, and maintains those beneath
it in a state of permanent distension. But the arteries, in spite of its
pressure, and under the force and impulse of the heart, send on the blood
from the internal parts of the body to the parts beyond the ligature. And
herein consists the difference between the tight and the medium ligature,
that the former not only prevents the passage of the blood in the veins,
but in the arteries also; the latter, however, whilst it does not prevent
the force of the pulse from extending beyond it, and so propelling the
blood to the extremities of the body, compresses the veins, and greatly or
altogether impedes the return of blood through them. Seeing,
therefore, that the moderately tight ligature renders the veins turgid and
distended, and the whole hand full of blood, I ask, whence is this? Does
the blood accumulate below the ligature coming through the veins, or
through the arteries, or passing by certain hidden porosities? Through the
veins it cannot come; still less can it come through invisible channels;
it must needs, then, arrive by the arteries, in conformity with all that
has been already said. That it cannot flow in by the veins appears plainly
enough from the fact that the blood cannot be forced towards the heart
unless the ligature be removed; when this is done suddenly all the veins
collapse, and disgorge themselves of their contents into the superior
parts, the hand at the same time resumes its natural pale colour, the
tumefaction and the stagnating blood having disappeared. Moreover,
he whose arm or wrist has thus been bound for some little time with the
medium bandage, so that it has not only got swollen and livid but cold,
when the fillet is undone is aware of something cold making its way
upwards along with the returning blood, and reaching the elbow or the
axilla. And I have myself been inclined to think that this cold blood
rising upwards to the heart was the cause of the fainting that often
occurs after bloodletting: fainting frequently supervenes even in robust
subjects, and mostly at the moment of undoing the fillet, as the vulgar
say, from the turning of the blood. Farther,
when we see the veins below the ligature instantly swell up and become
gorged, when from extreme tightness it is somewhat relaxed, the arteries
meantime continuing unaffected, this is an obvious indication that the
blood passes from the arteries into the veins, and not from the veins into
the arteries, and that there is either an anastomosis of the two orders of
vessels, or porosities in the flesh and solid parts generally that are
permeable to the blood. It is farther an indication that the veins have
frequent communications with one another, because they all become turgid
together, whilst under the medium ligature applied above the elbow; and if
any single small vein pricked with a lancet, they all speedily shrink, and
disburthening themselves into this they subside almost simultaneously. These
considerations will enable anyone to understand the nature of the
attraction that is exerted by ligatures,; and perchance of fluxes
generally; how, for example, when the veins are compressed by a bandage of
medium tightness applied above the elbow, the blood cannot escaped whilst
it still continues to be driven in, by the forcing power of the heart, by
which the parts are of necessity filled, gorged with blood. And now should
it be otherwise? Heat and pain and a vacuum draw, indeed; but in such wise
only that parts are filled, not preternaturally distended or gorged, and
not so suddenly and violently overwhelmed with the charge of blood forced
in upon them, that the flesh is lacerated and the vessels ruptured.
Nothing of the kind as an effect of heat, or pain, or the vacuum force, is
either credible or demonstrable. Besides,
the ligature is competent to occasion the afflux in question without
either pain, or heat, or a vacuum. Were pain in any way the cause, how
should it happen that, with the arm bound above the elbow, the hand and
fingers should swell below the bandage, and their veins become distended?
The pressure of the bandage certainly prevents the blood from getting
there by the veins. And then, wherefore is there neither swelling nor
repletion of the veins, nor any sign or symptom of attraction or afflux,
above the ligature’? But this is the obvious cause of the preternatural
attraction and swelling below the bandage, and in the hand and fingers,
that the blood is entering abundantly, and with force, but cannot pass out
again. Now
is not this the cause of all tumefaction, as indeed Avicenna has it, and
of all oppressive redundancy in parts, that the access to them is open,
but the egress from them is closed? Whence it comes that they are gorged
and tumefied. And may not the same thing happen in local inflammations,
where, so long as the swelling is on the increase, and has not reached its
extreme term, a full pulse is felt in the part, especially when the
disease is of the more acute kind, and the swelling usually takes place
most rapidly. But these are matters for after discussion. Or does this,
which occurred in my own case, happen from the same cause. Thrown from a
carriage upon one occasion, I struck my forehead a blow upon the place
where a twig of the artery advances from the temple, and immediately,
within the time in which twenty beats could have been made, I felt a
tumour the size of an egg developed, without either heat or any great
pain: the near vicinity of the artery had caused the blood to be effused
into the bruised part with unusual force and velocity. And
now, too, we understand why in phlebotomy we apply our ligature above the
part that is punctured, not below it; did the flow come from above, not
from below, the constriction in this case would not only be of no service,
but would prove a positive hinderance; it would have to be applied below
the orifice, in order to have the flow more free, did the blood descend by
the veins from superior to inferior parts; but as it is elsewhere forced
through the extreme arteries into the extreme veins, and the return in
these last is opposed by the ligature, so do they fill and swell, and
being thus filled and distended, they are made capable of projecting their
charge with force, and to a distance, when any one of them is suddenly
punctured; but the ligature being slacked, and the returning channels thus
left open, the blood forthwith no longer escapes, save by drops; and, as
all the world knows, if in performing phlebotomy the bandage be either
slackened too much or the limb be bound too tightly, the blood escapes
without force, because in the one case the returning channels are not
adequately obstructed; in the other the channels of influx, the arteries,
are impeded. |