The 16th and 17th centuries transformed medical understanding, shifting from reverence for ancient authorities to reliance on direct observation and empirical inquiry. For over a millennium, European medical practice was governed by the doctrines of the second-century Roman physician Galen. Galen’s system was based on the theory of the four humors—blood, phlegm, yellow bile, and black bile—which were believed to determine health based on their balance. Disease was treated by aggressive measures like bloodletting or purging to restore this equilibrium.
This era saw the rise of a new intellectual movement that championed experimentation over established dogma. The scientific method, emphasizing hypothesis and verification, began to penetrate the study of the human body, slowly eroding the foundations of Galenic tradition. Scholars started performing their own investigations, moving away from the passive acceptance of old texts and laying the groundwork for modern medicine.
Revolutionizing Human Anatomy
The study of human structure was dramatically overhauled in the mid-16th century by rejecting the tradition of relying on Galen’s anatomical descriptions. Galen’s authoritative work was largely based on the dissection of animals, such as apes and pigs, due to restrictions on human dissection in ancient Rome. This reliance meant his descriptions contained numerous inaccuracies that had been taught as fact for over 1,300 years.
The Belgian physician Andreas Vesalius challenged this flawed system by personally performing human dissections. His seminal work, De Humani Corporis Fabrica (On the Fabric of the Human Body), published in 1543, became the new standard for anatomical knowledge. Vesalius systematically detailed the human body, based entirely on his firsthand observations.
He documented approximately 300 errors in Galen’s teachings, including the structure of the human breastbone, which Galen incorrectly described as having seven parts, like that of an ape. The Fabrica was revolutionary not only for its empirical content but also for its presentation, featuring over 200 detailed woodcut illustrations. Vesalius’s work established that anatomical knowledge must be gained through direct, hands-on engagement with the human body, paving the way for investigations into how these structures functioned.
Understanding Blood Circulation
Following the revolution in anatomical structure came a parallel breakthrough in physiological function with the discovery of the complete, closed-circuit flow of blood. Galenic physiology taught that blood was continuously generated in the liver and consumed by the organs, passing through invisible pores in the heart’s septum.
The English physician William Harvey fundamentally overturned this concept with his 1628 publication, Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, known as De Motu Cordis. Harvey used observation, vivisection, and quantitative reasoning to demonstrate that the heart acts as a muscular pump. He observed that the heart’s contraction actively expels blood into the arteries.
Harvey calculated that the volume of blood pumped by the heart in a short period far exceeded the total body mass, making continuous consumption and replacement mathematically impossible. His experiments with ligatures on limbs demonstrated that blood flowed away from the heart in arteries and returned through veins, proving the circulatory nature of the system.
This proof of ceaseless circulation redefined the body as a machine governed by physical laws. Though he could not see the microscopic capillaries, Harvey correctly inferred their existence to complete the circuit, marking the beginning of modern physiology.
Advancements in Surgical Practice and Wound Care
Practical surgery experienced a significant, more humane shift, particularly in the chaotic environment of the battlefield. Before this period, surgical practices were often brutal, and traditional treatments frequently caused more suffering than the injury itself. The treatment of gunshot wounds, which became increasingly common, typically involved pouring boiling oil into the wound cavity to cauterize and supposedly “detoxify” the tissue.
The French military surgeon Ambroise Paré, who began his career as a barber-surgeon, made immediate and impactful changes in the mid-16th century. During the 1537 siege of Turin, Paré ran out of boiling oil and was forced to improvise a less harsh treatment. He applied a soothing balm made of egg yolk, rose oil, and turpentine to the wounds of some soldiers.
The next morning, he observed that patients treated with the gentle dressing were resting comfortably, while those who received traditional cauterization suffered from extreme pain, fever, and swelling. Paré immediately abandoned boiling oil, advocating for the gentler method of simple dressings. His innovations also extended to amputations, where he revived the ancient Roman technique of using silk threads, or ligatures, to tie off severed arteries and veins to control bleeding. This practice replaced the painful and often ineffective method of applying a red-hot iron to the stump to achieve hemostasis.
Paré’s dedication to patient-centric care and evidence-based improvisation earned him the title of the father of modern surgery. His famous quote, “I dressed him, God cured him,” reflected his humility and his focus on providing the best possible practical care. These changes dramatically improved patient outcomes and reduced suffering from common surgical procedures and war injuries.
The Dawn of Microscopic Observation
The final major advance of the 17th century was the introduction of the microscope, a new tool that expanded medical observation beyond the limits of the naked eye. Developed from glass lenses used in spectacles, the microscope opened a previously invisible world, laying the conceptual groundwork for pathology and microbiology. While the instrument did not immediately change medical treatment, it initiated a new phase of scientific exploration into the very small.
Robert Hooke, an English scientist, published his groundbreaking work Micrographia in 1665, featuring detailed drawings of objects viewed through his compound microscope. Hooke observed the porous structure of cork and coined the term “cell” to describe the box-like compartments, likening them to the small rooms inhabited by monks. He also provided the first published depiction of a microorganism, a microfungus, documenting life on a miniature scale.
A few years later, the Dutch tradesman Antonie van Leeuwenhoek made startling discoveries using single-lens microscopes he ground himself, achieving magnifications up to 250 times. Leeuwenhoek was the first person to observe and accurately describe single-celled organisms, which he referred to as “animalcules,” found in pond water and various substances. His meticulous observations, communicated to the Royal Society of London, also included descriptions of bacteria, protozoa, red blood cells, and sperm cells. This new technology effectively set the stage for the germ theory of disease, although its full medical significance would not be realized until centuries later.