80000 IMPUESTOS, REGALIAS Y TASAS

Long before the 16th century, various cultures had been curi- ous about the purpose of the blood, how it was made, and where it might possibly go within the body. (The assumption was that it was continually being made and consumed, and that was one of the reasons why bloodletting seemed like a sensible option. If blood was made continuously, there was no reason to worry about removing some of it.)

The early Greeks explored many elements in efforts to better understand how the body worked. As early as the fourth century

b.c.e., Aristotle studied human anatomy and located the blood ves-

sels. Then Praxagoras of Cos (fourth century b.c.e.) noted that

arteries were different from veins and put forth the view that air (pneuma) circulated through the arteries, while blood circulated through the veins. Two more scientists came along and further completed this picture. Praxagoras’s student, Herophilus of Chal- cedon (335–280 b.c.e.) reached a different conclusion, believing

arteries carried blood not air. He also studied the body’s pulse

William Harvey, first to describe the  circulation  of  the  blood (The Yorck Project)

rate and using a “water clock” developed ways to document pulse strength and rhythm. Thirty years later, another gifted medical practitioner, Erasistratus of Ceos (304–250 b.c.e.), decided that

blood in the body must move similarly to the way sap moves in trees. He mapped the veins and arteries and concluded that the heart functioned like a pump to move the blood around.

The theories developed by Herophilus and Erasistratus were very advanced for their time, and had other physicians and sci- entists used the theories introduced by these Greek physicians as stepping-stones toward better understanding of blood circulation, they would have arrived at a more accurate understanding much sooner. As it happened, Herophilus and Erasistratus’s enlightened realizations were not taken seriously. Only fragments of their writing survived to be passed on to other scientists, and their ideas were also trounced by Galen, who dominated medicine from the second century onward. Galen was highly critical of Herophilus and Erasistratus for not adhering to Hippocrates’ teachings, and Galen himself was developing his own theories about the blood that he wanted others to believe.

Galen worked primarily on animals (though his written mate- rials never specified this), and he determined that there were two types of blood in humans: the fresh and well-nourished blood (dark red) that traveled via the veins to the right auricle (upper cham- ber) of the heart and then to the right ventricle (lower chamber) where it passed through the septum of the heart to the left side, mixing with arterial blood that had picked up air from the lungs, making it brighter and thinner. He believed that the blood in the veins was created in the liver from “nutritious substances” (food), and its purpose was to nourish the organs and tissues where it was eventually “consumed” by the organs and tissues. The job of the arteries was to take blood from the heart to the brain where impurities were filtered out and discharged. Galen also taught that the blood moved because the arterial system could contract, caus- ing the blood to ebb and flow like the sea. Galen also identified the vascular network rete mirabile that he said was in the neck of all living things. (This network does not exist in humans, so this

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The ScienTific RevoluTion and Medicine

was one of the prime ways that scientists came to understand that Galen had not studied human bodies.)

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As early as the 13th century, Islamic physicians had developed a better understanding of how the heart and circulatory system worked. The Western world, however, was unaware of these gains until a 20th-century discovery. In 1924, an Egyptian physician Dr. Muhyi ad-Din at-Tatawi wrote his thesis on some little-known writings of physician Ibn an-Nafis (1210–80). About 40 years later, this doctoral thesis came to the attention of the historian Max Meyerhof, who read the thesis to learn what Ibn an-Nafis believed. Though no contemporary writers of his time seemed to have picked up on Ibn an-Nafis’s findings (though some may who have not yet been translated), Ibn an-Nafis had come to understand how the blood travels through the body. Acknowledging Galen’s theory, Ibn an-Nafis agreed with Galen that the left ventricle con- tained vital spirit while the right ventricle contained blood, but he disagreed with Galen’s theory about the pores within the septum permitting blood and spirit to pass between the left and right sides of the heart. Ibn an-Nafis theorized that the blood needed to go from the right ventricle to the lungs to acquire air, and only then would it enter the left ventricle. This theory was correct, and it preceded what was learned later by those in the 16th century who were studying anatomy. Andreas Vesalius (see chapter 2) was the first to raise concern that the septum was too dense to permit blood to pass through, and one of his assistants Realdo Colombo of Cremona developed a theory concerning a pulmonary circula- tion system, which was further developed by his pupil Andreas Casalpinus. Even with these breakthroughs, however, these men still felt that the veins were the key to distributing blood through the body.

As it happened, Galen’s theories lived on for the next 1,500 years, because no one else had any ideas that were better (and because Ibn an-Nafis’s work was not translated until much later).

Throughout the Middle Ages, human dissections were still frowned upon, and if physicians did not accept Galen’s ideas, then they were left with other questions: If the liver did not cre- ate blood, what did the liver do? If food was not converted into blood in the liver, then where did the food go and what purpose did it serve? How were the tissues nourished if the tissues did not “consume” the blood?