William Harvey - "De Motu Cordis" (otherwise Known As "On The Motion of The Heart and Blood")

"De Motu Cordis" (otherwise Known As "On The Motion of The Heart and Blood")

Published in 1628 in the city of Frankfurt (host to an annual book fair that Harvey knew would allow immediate dispersion of his work), this 72 page book contains the matured account of the circulation of the blood. Opening with a simple but clear dedication to King Charles I, the quarto has 17 chapters which give a perfectly clear and connected account of the action of the heart and the consequent movement of the blood around the body in a circuit. Having only a mere lens at his disposal, Harvey was not able to reach the adequate images that were attained through such microscopes used by Leeuwenhoek; thus he had to resort to theory – and not practical evidence – in certain parts of his book. After the first chapter, which simply outlines past ideas and accepted rules regarding the heart and lungs, Harvey moves on to a fundamental premise to his treatise, stating that it was extremely important to study the heart when it was active in order to truly comprehend its true movement; a task which even he found of great difficulty, as he says:

"...I found the task so truly arduous... that I was almost tempted to think... that the movement of the heart was only to be comprehended by God. For I could neither rightly perceive at first when the systole and when the diastole took place by reason of the rapidity of the movement..."

This initial thought led Harvey's ambition and assiduousness to a detailed analysis of the overall structure of the heart (studied with less hindrances in cold-blooded animals). After this, Harvey goes on to an analysis of the arteries, showing how their pulsation depends upon the contraction of the left ventricle, while the contraction of the right ventricle propels its charge of blood into the pulmonary artery. Whilst doing this, the physician reiterates the fact that these two ventricles move together almost simultaneously and not independently like had been thought previously by his predecessors. This discovery was made while observing the heart of such animals as the eel and several other types of fish; indeed, the general study of countless animals was of utmost importance to the physician: among the ones already cited, one can add the study of the snail, the invisible shrimp, the chick before its hatching and even the pigeon. A digression to an experiment can be made to this note: using the inactive heart of a dead pigeon and placing upon it a finger wet with saliva, Harvey was able to witness a transitory and yet incontrovertible pulsation. He had just witnessed the heart's ability to recover from fatigue.

As early as the 17th century, William Harvey had already discerned the existence of the Ductus Arteriosus and explained its relative function. Here he says, "...in embryos, whilst the lungs are in a state of inaction, performing no function, subject to no movement any more than if they had not been present, Nature uses the two ventricles of the heart as if they formed but one for the transmission of the blood." However, the apex of Harvey's work is probably the eighth chapter, in which he deals with the actual quantity of blood passing through the heart from the veins to the arteries. Coming into conflict with Galen's accepted view of the liver as the origin of venous blood, Harvey estimated the capacity of the heart, how much blood is expelled through each pump of the heart, and the amount of times the heart beats in a half an hour. All of these estimates were purposefully low, so that people could see the vast amount of blood Galen's theory required the liver to produce. He estimated that the capacity of the heart was 1.5 imperial fluid ounces (43 ml), and that every time the heart pumps, 1/8 of that blood is expelled. This led to Harvey's estimate that about 1⁄₆ imperial fluid ounces (4.7 ml) of blood went through the heart every time it pumped. The next estimate he used was that the heart beats 1000 times every half an hour, which gave 10 pounds 6 ounces of blood in a half an hour, and when this number was multiplied by 48 half hours in a day he realized that the liver would have to produce 540 pounds of blood in a day.

Having this simple but essential mathematical proportion at hand – which proved the overall impossible aforementioned role of the liver – Harvey went on to prove how the blood circulated in a circle by means of countless experiments initially done on serpents and fish: tying their veins and arteries in separate periods of time, Harvey noticed the modifications which occurred; indeed, as he tied the veins, the heart would become empty, while as he did the same to the arteries, the organ would swell up.

This process was later performed on the human body (in the image on the right): the physician tied a tight ligature onto the upper arm of a person. This would cut off blood flow from the arteries and the veins. When this was done, the arm below the ligature was cool and pale, while above the ligature it was warm and swollen. The ligature was loosened slightly, which allowed blood from the arteries to come into the arm, since arteries are deeper in the flesh than the veins. When this was done, the opposite effect was seen in the lower arm. It was now warm and swollen. The veins were also more visible, since now they were full of blood. Harvey then noticed little bumps in the veins, which he realized were the valves of the veins, discovered by his teacher, Hieronymus Fabricius. Harvey tried to push blood in the vein down the arm, but to no avail. When he tried to push it up the arm, it moved quite easily. The same effect was seen in other veins of the body, except the veins in the neck. Those veins were different from the others – they did not allow blood to flow up, but only down. This led Harvey to believe that the veins allowed blood to flow to the heart, and the valves maintained the one way flow.

It is also important to state how Harvey had theorized the existence of capillaries: however, unable to discern them due to the aforementioned scarcity of instruments at this disposal, he was never truly capable of understanding how blood passed from the arterioles into the venules.