心血運動論
他證明血液沿著一條無盡的迴路循環——而心臟，就是驅動它的泵。
一千四百年來，醫生們都相信身體在不停地造出新血、再把它燒掉用盡。一道仔細的算術，證明了這絕無可能。
核心思想
在哈維之前，醫學遵循古希臘醫師蓋倫：血在肝臟裡造出，流向全身，像燃料一樣被用盡——一股單向的潮水，造出來，又被耗掉。哈維卻證明，血是沿著一條迴路行進的。心臟是一台泵；它把同一份血經動脈推出去，又經靜脈收回來，一圈又一圈，永不停息。
他一錘定音的一著，不是什麼新儀器——而是算術。他算出心臟每跳一次推出多少血，再乘以僅僅半小時裡的心跳次數，得到的血量，遠遠大於全身所有的血。身體絕無可能在這麼短的時間裡造出這麼多。所以，那必是同一份血，在循環。
它是怎麼來的
哈維曾在義大利偉大的解剖學重鎮帕多瓦求學，師從法布里修斯——後者注意到靜脈裡有一些小小的瓣膜，卻始終沒弄明白它們是做什麼的。回到倫敦、成為兩代國王御醫的哈維，花了多年解剖活體動物，看著一顆顆心臟跳動、變慢、停下，感覺著脈搏與那一次次擠壓合著拍子。
他看到那些瓣膜全都朝著一個方向——朝向心臟。他按壓自己手臂上的靜脈，看著它拒絕從錯誤的方向重新充盈。然後，他算了那道帳。1628 年，他把它寫進一本薄薄的拉丁文小書《心血運動論》。它被攻擊了許多年；有些同行認為他「腦子壞了」。但他活著看到它被接受。
它為什麼重要
一旦知道血在循環，下游的一切便成為可能：測量血壓，理解休克與心力衰竭，輸血，心臟直視手術，人工心肺機。但更深的革命，在於方法。哈維不是靠引用更古老的權威取勝的；他靠的是測量，讓數字推翻了十五個世紀的成見。現代醫學，正是這樣學會了思考。
可以這樣想像
想像一座城市的供水。蓋倫設想的是一眼泉水，被全城喝乾——水在源頭造出、在龍頭用盡，永遠需要更多。哈維看到的是真相：那是一套閉合的管路，心臟是它的泵，同一股水循環著流過每一戶人家，再流回來。那些單向的瓣膜，就是防止水倒流的止回閥。
它在知識譜系中的位置
哈維這本書（1628）站在近代科學革命的開端，比牛頓的《自然哲學的數學原理》（1687，亦在本館）早了一代人——後者在物理學裡做了同樣的一步：以測量與機制，取代承襲來的權威。在醫學裡，它開啟了一條貫穿巴斯德與弗萊明（其青黴素亦在本館）的脈絡——治病這件事，被緩緩地變成一門科學。哈維自己那條迴路的最後一塊——微血管——則由馬爾比基在 1661 年用新生的顯微鏡找到。
獻詞 · 致查理一世
The heart of animals is the foundation of their life, the sovereign of everything within them, the sun of their microcosm, that upon which all growth depends, from which all power proceeds.
第八章 · 如環之運動
I began to think whether there might not be a motion, as it were, in a circle. Now this I afterwards found to be true; and I finally saw that the blood, forced by the action of the left ventricle into the arteries, was distributed to the body at large, and its several parts, in the same manner as it is sent through the lungs, impelled by the right ventricle into the pulmonary artery, and that it then passed through the veins and along the vena cava, and so round to the left ventricle in the manner already indicated.
第九章 · 數量的論證
Whence it is to be inferred, that if at one stroke the heart in man, the ox or the sheep, ejects but a single drachm of blood, and there are one thousand strokes in half an hour, in this interval there will have been ten pounds five ounces expelled: were there with each stroke two drachms expelled, the quantity would of course amount to twenty pounds and ten ounces; were there half an ounce, the quantity would come to forty-one pounds and eight ounces; and were there one ounce it would be as much as eighty-three pounds and four ounces; the whole of which, in the course of one half hour, would have been transfused from the veins to the arteries.
第十四章 · 結論
It is absolutely necessary to conclude that the blood in the animal body is impelled in a circle, and is in a state of ceaseless motion; that this is the act or function which the heart performs by means of its pulse; and that it is the sole and only end of the motion and contraction of the heart.