Galen of Pergamon (c. 129–210 AD) became the single most influential medical figure of the ancient world. His anatomical and physiological writings formed the foundation of Western medical understanding for over 1,300 years, dominating practice from the fall of Rome through the Middle Ages. The sheer volume and systematic nature of his work established a benchmark for medical scholarship that was rarely challenged for centuries. His anatomical theories were revered for their supposed accuracy concerning the human body.
The Historical Barrier to Human Dissection
Galen practiced medicine during a time when direct investigation of the human form through dissection was prohibited across the Roman Empire. This restriction was rooted in a strong cultural and legal opposition to the violation of the human body after death. Roman society generally viewed the integrity of the corpse as sacrosanct, making anatomical dissection a deeply taboo and forbidden practice.
Centuries earlier, human dissection had been briefly and systematically performed in Alexandria by figures such as Herophilus and Erasistratus. However, by Galen’s time, this period of anatomical liberty had long passed, and the prohibition was strictly enforced. This societal barrier explains why Galen never had the opportunity to verify his theories on human cadavers. This limitation forced him to seek alternative subjects to build his comprehensive model of human anatomy.
Anatomical Studies Based on Animal Models
To circumvent the prohibition on human dissection, Galen diligently applied his surgical and observational skills to a variety of animals, extrapolating his findings to the human form. He considered the Barbary macaque, a tailless monkey, to be the animal whose internal structures most closely resembled those of a human being. His most comprehensive anatomical reports were primarily based on the dissection of these primates, which he believed offered the best available proxy for human viscera, musculature, and nervous systems.
Galen also routinely used pigs and oxen for his anatomical demonstrations and physiological experiments, often performing vivisections, or dissections on living animals. He famously used pigs to demonstrate the function of the recurrent laryngeal nerve, which controls the voice, by cutting the nerve and observing the immediate cessation of the animal’s squeals.
Furthermore, his experience as a physician to the gladiators in Pergamon provided him with an unparalleled opportunity to observe human anatomy directly through treating severe wounds and injuries. This exposure allowed him to form accurate conclusions about the location of certain organs and the structure of the human skeleton.
His method involved meticulously dissecting these animal subjects and then, based on the principle of biological similarity, projecting the observed structures onto the human body. While this approach allowed him to make significant discoveries about the nervous system, such as the relationship between the spinal cord and movement, it also introduced fundamental inaccuracies where animal and human anatomy diverged.
The Specific Anatomical Errors Introduced by Galen’s Work
The reliance on animal models inevitably led to the inclusion of several specific anatomical features that do not exist in humans, which were perpetuated by Galen’s authority for over a millennium. One of the most significant errors was his description of the rete mirabile, or “wonderful net,” a complex network of blood vessels at the base of the brain. This structure is present in some ungulates, such as oxen and sheep, but Galen mistakenly assumed it was also present in humans, assigning it the function of turning “vital spirits” into “animal spirits.”
His understanding of the circulatory system was also fundamentally flawed due to animal analogies. Galen believed that the venous blood originated in the liver and that blood passed from the right side of the heart to the left through invisible pores in the interventricular septum. This dual-blood system was based on his observation of animal hearts and ignored the pulmonary circulation later discovered in humans. In another instance, he described the human liver as having five distinct lobes, a structure more typical of a pig’s liver, not the four-lobed human organ.
Furthermore, his musculoskeletal descriptions contained errors, such as the belief that the human mandible, or lower jawbone, was composed of two separate bones, a feature found in dogs and other animals he dissected. These discrepancies were finally exposed and corrected in the Renaissance, most notably by anatomist Andreas Vesalius in 1543. Vesalius’s work, which was based on direct human dissection, demonstrated that Galen’s authority, though immense, had been built on an imperfect foundation of comparative anatomy.