さて，それだけ見たあとは，心臓の運動を説明するには次のことを言えば足りる。つまり，心室が血液でいっぱいになっていないと，血液は必ず，大静脈から右心室，静脈性動脈から左心室へと流れる。この二つの管はつねに血液で満たされていて，心臓に向かって開いているその口は，その時ふさぐことができないからである。ところがこうして二滴の血液が，各心室に一滴ずつ入っていくと，入る口はひじように大きく，通ってきた管は血液でいっぱいだから，きわめて大きな滴になるほかはない。心室には熱があるため，すぐに密度が低くなり膨張する。それによって心臓全体を膨らませ，いま通ってきた二つの管の入り口にある五つの小さな扉を押して閉じ，それ以上の血液が心臓に流れ込むのを止める。そして血液の滴はますます密度を低下させていき，ほかの二つの管の入り口にある六つの小さな扉を押し開け，そこから出ていく。このようなやり方で，動脈性静脈と大動脈のあらゆる支管を心臓とほとんど同時に膨らませる。動脈に入った血液はそこで冷えるから，心臓はそのあとすぐに，これらの動脈と同じく収縮し，動脈の六つの小扉はまた閉じる。そして大静脈と静脈性動脈の五つの扉がふたたび開いて，次の二滴の血液を通し，それが前の二瓶とまったく同じょぅに，心臓と動脈をふたたび膨らませる。こうして心臓に入る血液は，心耳とよばれる例の二つの袋を通るために，心耳の運動は心臓の運動と逆になり，心臓が膨らむと心耳は収縮する。なお，数学的論証の力を知らす，真の理由と真らしい理由とを区別するのに慣れていない人たちが，検討もせずにこのことを否定する危険のないように，次のことを注意しておきたい。つまり，わたしがいま説明した運動は，心臓のなかで見て確かめることができる諸器官のただの配置，指で感じることができる熱，実験によって知ることができる血液の性質から，必然的に帰結するのであり，それは時計の運動が，分銅と歯車の力，位置，形から生まれる結果であるのと同じだということである。
For, after these things, it is not necessary
for me to say anything more with a view to
explain the motion of the heart, except that
when its cavities are not full of blood,
into these the blood of necessity flows,?from
the hollow vein into the right, and from
the venous artery into the left; because
these two vessels are always full of blood,
and their orifices, which are turned towards
the heart, cannot then be closed. But as
soon as two drops of blood have thus passed,
one into each of the cavities, these drops
which cannot but be very large, because the
orifices through which they pass are wide,
and the vessels from which they come full
of blood, are immediately rarefied, and dilated
by the heat they meet with. In this way they
cause the whole heart to expand, and at the
same time press home and shut the five small
valves that are at the entrances of the two
vessels from which they flow, and thus prevent
any more blood from coming down into the
heart, and becoming more and more rarefied,
they push open the six small valves that
are in the orifices of the other two vessels,
through which they pass out, causing in this
way all the branches of the arterial vein
and of the grand artery to expand almost
simultaneously with the heart?which immediately
thereafter begins to contract, as do also
the arteries, because the blood that has
entered them has cooled, and the six small
valves close, and the five of the hollow
vein and of the venous artery open anew and
allow a passage to other two drops of blood,
which cause the heart and the arteries again
to expand as before. And, because the blood
which thus enters into the heart passes through
these two pouches called auricles, it thence
happens that their motion is the contrary
of that of the heart, and that when it expands
they contract. But lest those who are ignorant
of the force of mathematical demonstrations,
and who are not accustomed to distinguish
true reasons from mere verisimilitudes, should
venture, without examination, to deny what
has been said, I wish it to be considered
that the motion which I have now explained
follows as necessarily from the very arrangement
of the parts, which may be observed in the
heart by the eye alone, and from the heat
which may be felt with the fingers, and from
the nature of the blood as learned from experience,
as does the motion of a clock from the power,
the situation, and shape of its counter-weights
and wheels.