Galen (Greek: Γαληνός, Galēnos; Latin: Claudius Galenus; AD 129 –ca. 200 or 216) of Pergamum was a prominent ancient Greek physician, whose theories dominated Western medical science for over a millennium. The forename "Claudius", absent in Greek texts, was first documented in texts from the Renaissance.

Galen was born in Pergamum, Mysia (now Bergama, Turkey). The son of the wealthy architect Nicon, he had eclectic interests—agriculture, architecture, astronomy, astrology, philosophy —before finally concentrating on medicine. By the age of twenty, he had served for four years in the local temple as a therapeutes ("attendant" or "associate") of the god Asclepius. Although Galen liked to study the human body, dissection of human corpses after death was against Roman law, so instead he used pigs, apes, and other animals. The legal limitations forced on him led to quite a number of mistaken ideas about the body. For instance, he thought a group of blood vessels near the back of the brain, the rete mirabile, was common in humans, but it is so only in animals. After his father's death in 148 or 149, he left Pergamum to study in Smyrna, Corinth, and Alexandria for the next twelve years. In 157 Galen returned to his native city, where he worked for three or four years as a physician in a gladiator school. During this time he gained much experience with treating trauma and especially wounds, which he later called "windows into the body".

Galen performed many audacious operations—including brain and eye surgeries— that were not tried again for almost two millennia. To perform cataract surgery, he would insert a long needle-like instrument into the eye behind the lens; he would then pull the instrument back slightly to remove the cataract. The slightest slip could have caused permanent blindness.

Galen moved to Rome in 162. There he lectured, wrote extensively, and performed public demonstrations of his anatomical knowledge. He soon gained a reputation as an experienced physician, attracting to his practice a large number of clients. Among them was the consul Flavius Boethius, who introduced him to the imperial court, where he became a physician to Emperor Marcus Aurelius. Despite being a member of the court, Galen reputedly shunned Latin, preferring to speak and write in his native Greek, a tongue that was actually quite popular in Rome. The prestigious physician would go on to treat Roman luminaries such as Lucius Verus, Commodus, and Septimius Severus. However, in 166 Galen returned to Pergamum again, where he lived until he went back to Rome for good in 169.

Galen spent the rest of his life at the Roman imperial court, where he was given leave to write and experiment. He performed vivisections of numerous animals to study the function of the kidneys and the spinal cord. His favorite animal subject was the Barbary ape.

It has been reported that Galen employed twenty scribes to write down his words. In 191, a fire in the Temple of Peace destroyed some of his records. Because of a reference in the 10th century Suda lexicon, the year of Galen's death has traditionally been placed at around 200. However, since some scholars argue that textual evidence shows Galen writing as late as 207, they contend that he lived longer, the latest year proposed being 216.

The translation c.830-870 of 129 works of Galen into Arabic by Hunayn ibn Ishaq and his assistants, and in particular Galen's insistence on a rational systematic approach to medicine, set the template for Islamic medicine, which rapidly spread throughout the Arab Empire. However, as the title "Doubts on Galen" of a book by Muhammad ibn Zakarīya Rāzi (Rhazes) (d. 925) makes clear, as well as the writings of Ibn al-Nafis, the works of Galen were not taken on unquestioningly, but as a challengeable basis for further enquiry. A strong emphasis on experimentation and empiricism led to new results and new observations, which were contrasted and combined with those of Galen by writers such as Razi, al-Majusi, Ibn Sina (Avicenna) and Ibn al-Nafis.

Later, in medieval Europe, Galen's writings on anatomy became the mainstay of the medieval physician's university curriculum along; but they suffered greatly from stasis and intellectual stagnation. In the 1530s, however, Belgian anatomist and physician Andreas Vesalius took on a project to translate many of Galen's Greek texts into Latin. Vesalius's most famous work, De humani corporis fabrica, was greatly influenced by Galenic writing and form. Seeking to revive Galen's methods and outlook, Vesalius turned to human cadaver dissection as an evolution of Galen's natural philosophy. Galen's writings enjoyed a revival at the hands of Vesalius, who promoted Galen and expounded on him through books and hands-on demonstrations. Since most of Galen's writings were also translated into Arabic, the Middle East knows and reveres him as "Jalinos". Galen, identified venous (dark red) and arterial (brighter and thinner) blood, each with distinct and separate functions. Venous blood was thought to originate in the liver and arterial blood in the heart; the blood flowed from those organs to all parts of the body where it was consumed.

Galen's emphasis on blood-letting as a remedy for almost any ailment remained influential until right through into the Nineteenth Century.

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*Galenic formulation *Timeline of medicine and medical technology

*Galen University *Galen *Galen, University of Dayton *Galen: A Biographical Sketch *Greek Biology and Medicine by Henry Osborn Taylor (1922) *Greek Biology and Medicine by Henry Osborn Taylor (1922): Chapter 5 - "The Final System: Galen" *Galen and the Greek-Hellenic History of Medicine *Misconceptions