しかし静脈の血液は，こうして絶えず心臓に流れ込むのに，どうして酒れてしまわないのか，また動脈は，心臓を通るすべての血液がそこへ流れていくのに，どうして血液であふれることがないのか，と問われるなら，イギリスのある医師によってすでに書かれたこと以外を答える必要はない。この医師は，この方面で先鞭をつけ，次のことを最初に説いた人として，称賛をささげなければならない。すなわち，動脈の末端にたくさんの微細な通路があって，動脈が心臓から受けとった血液はそこを通って静脈の小さな枝に入り，そこからまた心臓に向かう，したがって血液の流れは永続的に繰り返される循環にほかならないということ。このことをかれは，外科医たちの普通の実験によって，実にうまく証明している。外科医たちは腕を，静脈を切開した個所の上の方で中ぐらいの強さで縛り，縛らなかったときょりも多量の血液が出るようにする。そしてもし下のほう，つまり手と，切開した個所とのあいだを縛るか，あるいは上のほうをたいへん強く縛ったならば「まったく反対のことが起こる。なぜなら，明らかに，中ぐらいの強さの縛り方だと，すでに腕のなかにある血液が静脈を通って心臓のほうへ戻ることを妨げることはできるが，それでも血液が心臓から動脈を通って絶えず新たにやってくるのを妨げないからだ。動脈は，静脈よりも深いところに位置し，動脈の膜のほうが硬くて圧迫しにくいし，また心臓から来る血液は，動脈を通って手の方へ行くほうが，手から静脈を通って心臓へ戻るときよりもずっと強い力で流れるからである。そしてこの動脈血が静脈の一つにある切り口を通って腕から流出するのだから，縛った個所の下の方，つまり腕の末端のほうに，必ず，血液が動脈から静脈に移ることができる通路がなければならないことになる。この医師はまた，血液の流れについて自分の説くところを，ある種の小さな弁によって，実にうまく証明している。これらの弁は，静脈にそっているいるな場所にあり，血液が身体の中心から末端へ流れることを許さず，末端から心臓に戻ることだけを許すようにできている。さらにかれは，次のようなことを示す実験によっても，自説をうまく証明している。体内の血液すべては，ただ一本の動脈を切開するだけで，ごくわずかな時間でそこから流出してしまうのであり，たとえその動脈を心臓にごく近いところできつく縛り，その紐と心臓のあいだで切開しても変わりはない。したがって，そこから流れ出る血液が心臓以外のところから来ると想像する何の根拠もなくなる。
But if it be asked how it happens that the
blood in the veins, flowing in this way continually
into the heart, is not exhausted, and why
the arteries do not become too full, since
all the blood which passes through the heart
flows into them, I need only mention in reply
what has been written by a physician 1 of
England, who has the honour of having broken
the ice on this subject, and of having been
the first to teach that there are many small
passages at the extremities of the arteries,
through which the blood received by them
from the heart passes into the small branches
of the veins, whence it again returns to
the heart; so that its course amounts precisely
to a perpetual circulation. Of this we have
abundant proof in the ordinary experience
of surgeons, who, by binding the arm with
a tie of moderate straitness above the part
where they open the vein, cause the blood
to flow more copiously than it would have
done without any ligature; whereas quite
the contrary would happen were they to bind
it below; that is, between the hand and the
opening, or were to make the ligature above
the opening very tight. For it is manifest
that the tie, moderately straitened, while
adequate to hinder the blood already in the
arm from returning towards the heart by the
veins, cannot on that account prevent new
blood from coming forward through the arteries,
because these are situated below the veins,
and their coverings, from their greater consistency,
are more difficult to compress; and also
that the blood which comes from the heart
tends to pass through them to the hand with
greater force than it does to return from
the hand to the heart through the veins.
And since the latter current escapes from
the arm by the opening made in one of the
veins, there must of necessity be certain
passages below the ligature, that is, towards
the extremities of the arm through which
it can come thither from the arteries. This
physician likewise abundantly establishes
what he has advanced respecting the motion
of the blood, from the existence of certain
pellicles, so disposed in various places
along the course of the veins, in the manner
of small valves, as not to permit the blood
to pass from the middle of the body towards
the extremities, but only to return from
the extremities to the heart; and farther,
from experience which shows that all the
blood which is in the body may flow out of
it in a very short time through a single
artery that has been cut, even although this
had been closely tied in the immediate neighbourhood
of the heart, and cut between the heart and
the ligature, so as to prevent the supposition
that the blood flowing out of it could come
from any other quarter than the heart.