だがこのほかにも，血液のこうした運動の真の原因が，わたしの述べたとおりであることを証明するものがたくさんある。たとえば第一に，静脈から出る血液と動脈から出る血液とのあいだに認められる違いは，次のことから生じるとしか考えられない。つまり，血液は心臓を通るときに薄くなり，いわば蒸留されるため，心臓から出た直後すなわち動脈のなかにあるほうが，心臓に入る直前すなわち静脈のなかにあるよりも，きめ細かく，活力があり，熱い，ということだ。そしてそこに注意すれば，この違いがよく現れるのは心臓の近くだけであり，心臓からいちばん遠いところではそれほどでもないことが見てとれる。次に，動脈性静脈と大動脈を組織している膜が硬いことから，血液が静脈ょりも動脈にたいして強い力で当たっていることが十分に分かる。また左心室と大動脈が，右心室と動脈性静脈よりも広くて太いのはなぜだろうか。静脈性動脈の血液は，すでに心臓を経て，その後は肺に行くだけなので，大静脈から直接やってくる血液よりもきめ細かく，いっそう強くかつ容易に薄まるからにほかならない。また，血液はその性質が変わるにしたがって，心臓の熱によって薄められる度合いが，前より強くなったり弱くなったり，早くなったり遅くなったりすることを，もし医師たちが知らなかったら，かれらは脈をとって，いったい何を見抜くことができるのか。またこの熱がどのようにして身体のほかの部分に伝わるかを調べてみると，それは血液が心臓を通る際に温めなおされて，そこから全身に広がることによるのを認めねばならない。そのため，身体のどこかの部分から血液をとり除くと，熱を奪うことになる。そして心臓がたとえ灼けた鉄のように熱くても，それが絶えず新しい血を送り込むのでなかったら，いま温めているほどに，手足を温めるには至らない。またそこから，呼吸の真の効用は，肺に冷たい空気を十分に送り込むことであるのが分かる。心臓の右心室で薄められ，いわば蒸気に変えられて肺へきた血液は，その冷たい空気によって濃厚になり，ふたたび血液に変わって，それからまた左心室へ流入していくのである。そうしなければ，血液は心臓にある火の養分として役立つのにふさわしいものとなれない。それは次のことからも確かめられる。つまり，肺のない動物は心臓にも一つの心室しかないし，母の胎内に閉じ込められているあいだ肺を使えない胎児は，血液を大静脈から心臓の左心室へ流す穴と，肺を通らずに動脈性静脈から大動脈に血液を送り込む導管を持っている。次に消化作用も，もし心臓が動脈を通じて胃に熱を送らず，またそれと併せて，胃に入った養分の分解を助ける血液のいちばん流動的な部分のいくらかを送らなかったら，胃でいったいどのように消化がなされるのか。そしてこうした食物の汁を血液に変える作用も，食物の汁がおそらく一日に百回ないし二百回以上も心臓に行ったり来たりすることで蒸留されるのを考えれば，たやすく分かるのではないか。また栄養摂取と体内にあるさまざまな体液の産出を説明するには，血液が薄くなって心臓から動脈の末端へと流れていく力の働きを述べるだけで十分ではないか。すなわちこの力によって，血液のいくらかの部分は，それが到達した肢体の一部にとどまり，そこで自分の追い出した血液のほかの部分にとって代わる。そして血液が出合う微細な孔の位置，形状，大小に応じて，血液のある部分はほかの部分とはちがった場所へ流れ込む。だれでもいろいろな節を見たことがあるだろうが，節はいるいる違ったふうに穴があげられていて，さまざまな穀粒を選り分けるのに使われる。血液もそれと同じようになる。そして最後に，これらすべてのうちで最も注目すべきものは，動物精気の生成である。それはきわめて微細な息，あるいはむしろきわめて純粋で活気のある炎のようなもので，絶えず豊富に心臓から脳へのぼり，そこから神経を通って筋肉に入っていき，肢体のあらゆる部分に運動をあたえる。血液のなかで最も活動的で浸透力が強く，この精気を構成するのにいちばん適した部分が，ほかの場所ではなく脳に向かっていく原因としては，次のことだけ考えればよい。つまり，この精気を脳に運ぶ動脈は，ほかのあらゆる動脈ょりも，心臓からまつすぐに来る動脈であること，そして，心臓の左心室から出る血液の諸部分が脳に向かうときのように，たくさんのものが同時に同じ方向に動こうとしてその全部を入れる余地がない場合は，自然の法則と同一である力学の法則に従って，弱くて不活発な部分が強い部分によって押しのけられるはずで，そうして強い部分だけが脳に行く，ということである。
But there are many other circumstances which
evince that what I have alleged is the true
cause of the motion of the blood: thus, in
the first place, the difference that is observed
between the blood which flows from the veins,
and that from the arteries, can only arise
from this, that being rarefied, and, as it
were, distilled by passing through the heart,
it is thinner, and more vivid, and warmer
immediately after leaving the heart, in other
words, when in the arteries, than it was
a short time before passing into either,
in other words, when it was in the veins;
and if attention be given, it will be found
that this difference is very marked only
in the neighbourhood of the heart; and is
not so evident in parts more remote from
it. In the next place, the consistency of
the coats of which the arterial vein and
the greatly artery are composed, sufficiently
shows that the blood is impelled against
them with more force than against the veins.
And why should the left cavity of the heart
and the great artery be wider and larger
than the right cavity and the arterial vein,
were it not that the blood of the venous
artery, having only been in the lungs after
it has passed through the heart, is thinner,
and rarefies more readily, and in a higher
degree, than the blood which proceeds immediately
from the hollow vein? And what can physicians
conjecture from feeling the pulse unless
they know that according as the blood changes
its nature it can be rarefied by the warmth
of the heart, in a higher or lower degree,
and more or less quickly than before? And
if it be inquired how this heat is communicated
to the other members, must it not be admitted
that this is effected by means of the blood,
which, passing through the heart, is there
heated anew, and thence diffused over all
the body? Whence it happens, that if the
blood be withdrawn from any part, the heat
is likewise withdrawn by the same means;
and although the heart were as hot as glowing
iron, it would not be capable of warming
the feet and hands as at present, unless
it continually sent thither new blood. We
likewise perceive from this, that the true
use of respiration is to bring sufficient
fresh air into the lungs, to cause the blood
which flows into them from the right ventricle
of the heart, where it has been rarefied
and, as it were, changed into vapours, to
become thick, and to convert it anew into
blood, before it flows into the left cavity,
without which process it would be unfit for
the nourishment of the fire that is there.
This receives confirmation from the circumstance,
that it is observed of animals destitute
of lungs that they have also but one cavity
in the heart, and that in children who cannot
use them while in the womb, there is a hole
through which the blood flows from the hollow
vein into the left cavity of the heart, and
a tube through which it passes from the arterial
vein into the grand artery without passing
through the lung. In the next place, how
could digestion be carried on in the stomach
unless the heart communicated heat to it
through the arteries, and along with this
certain of the more fluid parts of the blood,
which assists in the dissolution of the food
that has been taken in? Is not also the operation
which converts the juice of food into blood
easily comprehended, when it is considered
that it is distilled by passing and repassing
through the heart perhaps more than one or
two hundred times in a day? And what more
need be adduced to explain nutrition, and
the production of the different humours of
the body, beyond saying, that the force with
which the blood, in being rarefied, passes
from the heart towards the extremities of
the arteries, causes certain of its parts
to remain in the members at which they arrive,
and there occupy the place of some others
expelled by them; and that according to the
situation, shape, or smallness of the pores
with which they meet, some rather than others
flow into certain parts, in the same way
that some sieves are observed to act, which,
by being variously perforated, serve to separate
different species of grain? And, in the last
place, what above all is here worthy of observation,
is the generation of the animal spirits,
which are like a very subtle wind, or rather
a very pure and vivid flame which, continually
ascending in great abundance from the heart
to the brain, thence penetrates through the
nerves into the muscles, and gives motion
to all the members; so that to account for
other parts of the blood which, as most agitated
and penetrating, are the fittest to compose
these spirits, proceeding towards the brain,
it is not necessary to suppose any other
cause, than simply, that the arteries which
carry them thither proceed from the heart
in the most direct lines, and that, according
to the rules of Mechanics, which are the
same with those of Nature, when many objects
tend at once to the same point where there
is not sufficient room for all, (as is the
case with the parts of the blood which flow
forth from the left cavity of the heart and
tend towards the brain,) the weaker and less
agitated parts must necessarily be driven
aside from that point by the stronger which
alone in this way reach it.