Aelius Galenus or Claudius Galenus (September AD 129 – 199/217; Greek: Γαληνός, Galēnos), better known as Galen of Pergamum (modern-day Bergama, Turkey), was a prominent Roman physician and philosopher of Greek origin, and probably the most accomplished medical researcher of the Roman period. His theories dominated and influenced Western medical science for well over a millennium. His account of medical anatomy was based on monkeys as human dissection was not permitted in his time, but it was unsurpassed until the printed description and illustrations of human dissections by Andreas Vesalius in 1543. Galen's account of the activities of the heart, arteries and veins endured until William Harvey established that the blood circulates with the heart acting as a pump in 1628. In the 19th century, student physicians would still read Galen to learn some concepts. Galen developed many nerve ligation experiments that supported the theory, which is still believed today, that the brain controls all the motions of the muscles by means of the cranial and peripheral nervous systems. Galen wrote a small work called "That the Best Physician is also a Philosopher", and he saw himself as being both, which meant grounding medical practice in theoretically sound knowledge or "philosophy" as it was called in his time. Galen was very interested in the dispute between Rationalist and Empiricist medical sects, and his use of direct observation, dissection and vivisection in medical training and as a way to ground medical practice can be understood as considering both of those perspectives and constructing a more complex and nuanced middle ground that avoided problems with each position.
He describes his early life in "On the affections of the mind". Born in September 129 AD,  his father Aelius Nicon was a wealthy patrician, an architect and builder, with eclectic interests including philosophy, mathematics, logic, astronomy, agriculture and literature. Galen describes his father as a "highly amiable, just, good and benevolent man". At that time Pergamon was a major cultural and intellectual centre, noted for its library (Eumenes II), second only to that in Alexandria   and attracted both Stoic and Platonic philosophers, to whom Galen was exposed at age 14. His studies also took in each of the principal philosophical systems of the time, including Aristotelian and Epicurean. His father had planned a traditional career for Galen in philosophy or politics and took care to expose him to literary and philosophical influences. However Galen states that in around 145, his father had a dream in which the God Asclepius (Aesculapius) appeared and commanded Nicon to send his son to study medicine. Again, no expense was spared, and following his earlier liberal education, at 16 he began studies at the prestigious local sanctuary or Asclepieum dedicated to Asclepius, God of medicine, as a θεραπευτής (therapeutes, or attendant) for four years. There he came under the influence of men like Aeschrion of Pergamon, Stratonicus and Satyrus. Asclepiea functioned as spas or sanitoria to which came the sick to seek the ministrations of the priesthood. The temple at Pergamon was eagerly sought by Romans in search of a cure. It was also the haunt of notable people such as Claudius Charax the historian, Aelius Aristeides the orator, Polemo the sophist, and Cuspius Rufinus the Consul. 
In 148, when he was 19, his father died, leaving him independently wealthy. He then followed the advice he found in Hippocrates' teaching  and travelled and studied widely including Smyrna (now Izmir), Corinth, Crete, Cilicia (now Çukurova), Cyprus and finally the great medical school of Alexandria, exposing himself to the various schools of thought in medicine. In 157, aged 28, he returned to Pergamon as physician to the gladiators of the High Priest of Asia, one of the most influential and wealthiest men in Asia. Over the four years there he learnt the importance of diet, fitness, hygiene and preventive measures, as well as living anatomy, and the treatment of fractures and severe trauma, referring to their wounds as "windows into the body". Only five deaths occurred while he held the post, compared to sixty in his predecessor's time, generally ascribed to his attention to their wounds. At the same time he pursued studies in theoretical medicine and philosophy. 
Galen provides accounts of his later life in Rome, in On Prognosis, and On his own Books. Στάσις (stasis, or political unrest) in Pergamon was probably the reason for Galen to leave Pergamon in 161, travelling in the Eastern Mediterranean studying the properties of minerals. His travels took him to Lemnos, Cyprus, and Palestinian Syria,before reaching Rome in August 162, aged 37, in the second year of the reign of the joint Emperors Marcus Aurelius and Lucius Verus. As a Greek in Rome, he faced cultural challenges, stiff competition and professional jealousy. 
One of his more famous patients was the peripatetic philosopher Eudemus, a friend of his father, and his former tutor. He recounts curing Eudemus of Quartan Fever in 162 (Praen 2:5)  This proved fortuitous, since during this illness, Eudemus was visited by Flavius Boethus, a former Consul and later Governor of Palestine (166-8),  Sergius Paulus, who became a Prefect, and Severus, uncle of the Emperor Lucius. They were Aristotelians and had heard of Galen's anatomical skills and were anxious to set up vivisection demonstrations, which they hoped would promote him (AA). Galen's skills in caring for Eudemus and his rigorous philosophical explanation of the pathology greatly enhanced his reputation in the upper circles of Rome. His straightforward didactic teaching of his patients led him to choose those with whom he could discourse as a clientele.  Word of how he gave Eudemus a prognosis earned disapproval from some Roman physicians such as Martianus (an Erasistratean), who compared it to divination. Providing a prognosis was not part of their tradition, unlike Galen and the Hippocratic school. Galen in turn criticised the Roman doctors for their relationship with rich patrons, ostentatious dress and belief that medicine could be learned quickly. Galen was fortunate in having the wise advice of Eudemus to guide him through the politics of Roman medicine and society, even warning him that he might be in danger of his life. 
At first reluctantly, but then with increasing vigour, Galen promoted Hippocratic teaching including venesection, then unknown in Rome. This was sharply criticised by Erasistrateans, who predicted dire outcomes, believing that it was not blood but Pneuma that flowed in the veins. Galen however staunchly defended venesection in his three books on the subject,  and in his demonstrations and public disputations. 
Galen's fame rested on his anatomical demonstrations, success with influential patrons where others had failed, his learning and his rhetoric. His background and wealth and friendship with Eudemus helped him advance in Roman society. However, Galen was not reluctant to show his contempt for the learning and ethics of his contemporaries in Rome, and his ambitiousness created enemies.
When he returned to Pergamon in August 166 he claimed he had departed Rome due to professional jealousy, although the outbreak of the Antonine Plague which accompanied the return of Lucius Verus' army in that year may have contributed to this.
He was recalled to Rome by the Emperors Marcus Aurelius and Lucius Verus to serve in the German wars, a task he did not relish, preferring to stay in Rome with Marcus Aurelius' son, Commodus. Amongst his clients was the Consul Flavius Boethus, who had introduced him to the imperial court, where he became personal physician to Marcus Aurelius and his son Commodus, returning to Rome on the death of Verus in 169. He later also served as physician to the Emperor Septimius Severus. His own writings are rich with anecdotes illustrating the heights of his fame. 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. Galen spent most of the rest of his life at the Roman imperial court, where he was given leave to write and experiment. The bulk of his output occurred during this period. For instance, On Prognosis was written in 177-8. He returned to Pergamon in the 190s.
The 11th century Suda lexicon states that Galen died at the age of 70, therefore about 199. However,there is a reference in Galen's treatise "On Theriac to Piso" (which may however be spurious) to events of 204. There are also statements in Arabic sources that he died at 87, after 17 years studying medicine and 70 practicing it, therefore about 217. Nutton believes that "On Theriac to Piso" is genuine, the Arabic sources are correct and that the Suda has erroneously interpreted the 70 years of Galen's career in the Arabic tradition as referring to his whole lifespan. Boudon-Millot more or less concurs and favours a date of 216.
Galen's works covered a wide range of topics, from anatomy, physiology, and medicine to logic and philosophy, both summarising what was known and adding his own observations. His writings pay homage to, amongst others, Plato, Aristotle and the Stoics, but above all to Hippocrates, whom he refers to as "divine" (θειότατος Ίπποκράτης Nat Fac III: 13). Thus much of his explanation of pathology relies on Hippocrates' humoral theories.
He proceeded by observation, deductive reasoning, and experimentation, such as his demonstration of the effect of ligating the ureters (Nat Fac I: 13), and the functions of the spinal cord. His medical practice drew on the biological theory and anatomical observations from Aristotle to the Alexandrians in addition to his own research. His therapeutics led him to travel widely gathering elements and plants. However his reasoning led him astray as much as it did to truth, such as his rejection of the role of the stomach wall in digestion (Nat Fac III: 4) and his concepts of specific attraction.
Galen's approach to colleagues and the state of knowledge was very forthright. He despised what he referred to as partisanship (Nat Fac I: 13), and was impatient with those with whom he disagreed, such as the Erasistrateans  and Asclepiadeans. (Nat Fac I: 17)  Another target of his scorn were the Methodists, abhorring their consideration of pathology in a vacuum, treating the disease not the patient, whereas he taught that vital processes in an organism had to be interpreted in relation to its environment. Other disputes were with the Atomists , and the Anatomists, arguing that the whole is far greater than the sum of the parts. His own personal credo was based on three branches of philosophy; logic, physics and ethics. (Opt Med) He wrote in a highly polished precise Attic style, using many words (such as haematopoietic) that have passed down to us in modern medical terminology, albeit with altered meaning.
Galen developed an interest in anatomy from his studies of Herophilus and Erasistratus, who had dissected the human body and even living bodies (vivisection).  Although Galen studied the human body, dissection of human corpses was against Roman law, so instead he performed vivisections on pigs, apes, and other animals (e.g. Nat Fac III: 8), to study the function of the kidneys and the spinal cord. In this study of comparative anatomy, he frequently dissected the Barbary Macaque and other primates, assuming their anatomy was basically the same as that of humans. 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, although it actually is absent in humans.
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, then pull the instrument back slightly to remove the cataract. The slightest slip could have caused permanent blindness.
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 produced more work than any author in antiquity,  and may have possibly written up to 600 treatises, although less than a third of his works have survived. His surviving work runs to around 3 million words. Carolus Kühn of Leipzig translated 122 of Galen's writings (1821-1833) and his edition, which is the most complete although flawed,  consists of the Greek text, with Latin translations, and runs to 22 volumes, 676 index pages, and is over 20,000 pages in length. More modern projects like the Corpus Medicorum Graecorum have still to match the Kühn edition. A digital version of the Galen's corpus is included in the Thesaurus Linguae Graecae a digital library of Greek literature started in 1972. Another useful modern source is the French Bibliothèque interuniversitaire de médicine (BIUM).
It has been reported that Galen employed 20 scribes to write down his words. In 191, a fire in the Temple of Peace destroyed many of his works, particularly treatises on philosophy. Because Galen's works were not translated into Latin in the ancient period, and because of the collapse of the Roman Empire in the West, the study of Galen, along with the Greek medical tradition as a whole, went into decline in Western Europe during the Early Middle Ages, when very few Latin scholars could read Greek. However, Galen and the ancient Greek medical tradition generally continued to be studied and followed in the Eastern Roman Empire, commonly known as the Byzantine Empire. All of the extant Greek manuscripts of Galen were copied by Byzantine scholars. In the Abbasid period (after 750 AD) Arab Muslims began to be interested in Greek scientific and medical texts for the first time, and had some of Galen's texts translated into Arabic, often by Syrian Christian scholars (see below). As a result some texts of Galen exist only in Arabic translation,  while others exist only in medieval Latin translations of the Arabic. In some cases scholars have even attempted to translate from the Latin or Arabic back into Greek where the original is lost.    So great was Galen's output in both quantity and authority that no single authoritative collection of his work exists, and controversy still exists as to the authenticity of a number of attributed works. The surviving Greek texts represent half of all the original Greek literature we have today. For some of the ancient sources, such as Herophilus, Galen's account of their work is all that survives. Even in his own time, forgeries and unscrupulous editions of his work were a problem, prompting him to write On his Own Books. Over the years many different titles have appeared for the same treatises. Sources are often in obscure and difficult to access journals or repositories. Forgeries in Latin, Arabic or Greek continued till the Renaissance. Consequently research on Galen's work is fraught with hazard.  Although written in Greek, by convention the works are referred to by Latin titles, and often by merely abbreviations of those.
Various attempts have been made to classify Galen's vast output. For instance Coxe (1846) lists a Prolegomena, or introductory books, followed by 7 classes of treatise embracing Physiology (28 vols.), Hygiene (12), Aetiology (19), Semeiotics (14), Pharmacy (10), Blood letting (4) and Therapeutics (17), in addition to 4 of aphorisms, and spurious works.
In his time, Galen's reputation as both physician and philosopher was legendary,  the Emperor Marcus Aurelius describing him as "Primum sane medicorum esse, philosophorum autem solum" (first among doctors and unique among philosophers Praen 14: 660). Other contemporary authors in the Greek world confirm this including Theodotus the Shoemaker, Athenaeus and Alexander of Aphrodisias. The 7th century poet George of Pisida going so far as to refer to Christ as a second and neglected Galen. Galen continued to exert an important influence over the theory and practice of medicine until the mid seventeenth century in the Byzantine and Arabic worlds and Europe. Hippocrates and Galen form important landmarks of 600 years of Greek medicine. AJ Brock describes them as representing the foundation and apex respectively.  A few centuries after Galen Palladius Iatrosophista in his commentary on Hippocrates, stated that Hippocrates sowed and Galen reaped. Thus Galen summarised and synthesised the work of his predecessors, and it is in Galen's words (Galenism) that Greek medicine was handed down to subsequent generations, such that Galenism became the means by which Greek medicine was known to the world. Frequently this was in the form of restating and reinterpreting, such as in Magnus of Nisibis' fourth century work on urine, which was in turn translated into Arabic.  Yet the full importance of his contributions was not appreciated till long after his death.  Galen's rhetoric and prolificity were so powerful as to convey the impression that there was little left to learn. The term Galenism has subsequently taken on both a positive and pejorative meaning as one that transformed medicine in late antiquity yet so dominated subsequent thinking as to stifle further progress. 
After the collapse of the Western Empire the study of Galen and other Greek works almost disappeared in the Latin West. In contrast, in the predominantly Greek-speaking eastern half of the Roman empire (Byzantium), many commentators of the subsequent centuries, such as Oribasius, physician to the emperor Julian who compiled a Synopsis in the 4th Century, preserved and disseminated Galen's works, making Galenism more accessible. Nutton refers to these authors as the "medical refrigerators of antiquity".  In late antiquity medical writing veered increasingly in the direction of the theoretical at the expense of the practical. Many authors merely debating Galenism. Magnus of Nisibis was a pure theorist, as was John of Alexandria and Agnellus of Ravenna with their lectures on Galen's De Sectis.  So strong was Galenism that other authors such as Hippocrates began to be seen through a Galenic lens, while his opponents became marginalised and other medical sects such as Asclepiadism slowly disappeared. 
Greek medicine was part of Greek culture, and Syrian Eastern Christians came in contact with it while the Eastern Roman Empire (Byzantium) ruled Syria and Western Mesopotamia, regions that were conquered from Byzantium in the seventh century by Arab Muslims. After 750 AD, Muslims had these Syrian Christians make the first translations of Galen into Arabic. From then on Galen and the Greek medical tradition in general became assimilated into the medieval and early modern Islamic Middle East. 
The first major translator of Galen into Arabic was the Syrian Christian Hunayn ibn Ishaq. Hunayn translated (c.830-870) 129 works of "Jalinos"  into Arabic. One of the Arabic translations, ‘Kitab ila Aglooqan fi Shifa al Amraz’, which is extant in the Library of Ibn Sina Academy of Medieval Medicine & Sciences, is a master piece of all literary works of Galen. It is a part of the Alexandrian compendium of Galen’s work. This manuscript of 10th century comprises of two parts that include details regarding various types of fevers (Humyat) and different inflammatory conditions of the body. More importantly, it includes details of more than 150 single and compound formulations of both herbal and animal origin. The book provides an insight into understanding the traditions and methods of treatment in Greek (Unani) and Roman era. In addition, this book provides a direct source for the study of more than 150 single and compound drugs used during Greeko-Roman period.
Galen's insistence on a rational systematic approach to medicine set the template for Islamic medicine, which rapidly spread throughout the Arab Empire. Arabic sources, such as Rhazes (Muhammad ibn Zakarīya Rāzi 865-925 AD), continue to be the source of discovery of new or relatively inaccessible Galenic writings.  As the title, Doubts on Galen by Rhazes implies, as well as the writings of physicians such as Ibn Zuhr (Avenzoar) and Ibn al-Nafis, the works of Galen were not taken on unquestioningly, but as a challengeable basis for further inquiry.
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 Rhazes, Ali ibn Abbas al-Majusi (Haly Abbas), Abu al-Qasim al-Zahrawi (Abulasis), Ibn Sina (Avicenna), Ibn Zuhr and Ibn al-Nafis. For example, the experiments carried out by Rāzi and Ibn Zuhr contradicted the Galenic theory of humorism, while Ibn al-Nafis' discovery of the pulmonary circulation contradicted the Galenic theory on the heart.
From the 11th century onwards, Latin translations of Islamic medical texts began to appear in the West, alongside the Salerno school of thought, and were soon incorporated into teaching at the universities of Naples and Montpellier. Galenism now took on a new unquestioned authority, Galen even being referred to as the "Medical Pope of the Middle Ages".  Constantine the African was amongst those who carried out translations of both Hippocrates and Galen from Arabic. In addition to the more numerous translations of Arabic texts in this period, there were a few translations of Galenic works directly from the Greek, such as Burgundio of Pisa's translation of De complexionibus. Galen's works on anatomy and medicine became the mainstay of the medieval physician's university curriculum, alongside Ibn Sina's The Canon of Medicine which elaborated on Galen's works. Unlike pagan Rome, Christian Europe did not exercise a universal prohibition of the dissection and autopsy of the human body and such examinations were carried out regularly from at least the 13th century. However, Galen's influence, as in the Arab world, was so great that when dissections discovered anomalies in Galen's anatomy, the physicians often tried to fit these into the Galenic system. An example of this is Mondino de Liuzzi, who describes rudimentary blood circulation in his writings but still asserts that the left ventricle should contain air.
The Renaissance and fall of the Byzantine Empire (1453) was accompanied by an influx of Greek scholars and manuscripts to the West, allowing direct comparison between the Arabic commentaries and the original Greek texts of Galen. This New Learning and the Humanist movement, particularly the work of Thomas Linacre, promoted litterae humaniores including Galen in the Latin scientific canon, De Naturalibus Facultatibus appearing in London in 1523. Debates on medical science now had two traditions, the more conservative Arabian and liberal Greek.  The more extreme liberal movements began to challenge the role of authority in medicine, as exemplified by Paracelsus symbolically burning the works of Avicenna and Galen at his medical school in Basle.  Nevertheless Galen's pre-eminence amongst the great thinkers of the millennium is exemplified by a 16th century mural in the refectory of the Great Lavra of Mt Athos. This depicts pagan sages at the foot of the Tree of Jesse, with Galen between the Sibyl and Aristotle. 
Galenism's final defeat came from a combination of the negativism of Paracelsus and the constructivism of the Italian Renaissance anatomists, such as Vesalius in the 16th century.  In the 1530s, Belgian anatomist and physician Andreas Vesalius took on a project to translate many of Galen's Greek texts into Latin. Vesalius' most famous work, De humani corporis fabrica, was greatly influenced by Galenic writing and form. Seeking to examine critically Galen's methods and outlook, Vesalius turned to human cadaver dissection as a means of verification. Galen's writings were shown to describe details present in monkeys but not in humans by Vesalius, who demonstrated Galen's limitations through books and hands-on demonstrations, despite fierce opposition from pro-Galenist orthodoxy, such as Jacobus Sylvius. Since Galen states that he is using observations of monkeys (human dissection was prohibited) to give an account of what the body looks like, Vesalius could portray himself as using Galen's approach of description of direct observation to create a record of the exact details of the human body, since he worked in a time when human dissection was allowed. Galen argued that monkey anatomy was close enough to humans for physicians to learn anatomy with monkey dissections and then make observations of similar structures in the wounds of their patients, rather than trying to learn anatomy only from wounds in human patients as students being trained by the Empiricist medical sect would. The examinations of Vesalius also disproved medical theories of Aristotle and Mondino de Liuzzi. One of the best known examples of Vesalius' overturning of Galenism was his demonstration that the interventricular septum of the heart was not permeable, as Galen had taught (Nat Fac III xv). Another convincing case where understanding of the body was extended beyond where Galen had left it came from these demonstrations of the nature of the circulation and the subsequent work of Andrea Cesalpino, Fabricio of Acquapendente and William Harvey.  Some Galenic teaching, such as his emphasis on bloodletting as a remedy for many ailments, however remained influential until well into the 1800s.
Galenic scholarship remains an intense and vibrant field, following renewed interest in his work, dating from the Realencyclopädie der Classischen Altertumswissenschaft. 
(Commentary on Hippocrates' On the Nature of Man; On the Natural Faculties; Exhortation to Study the Arts: To Menodotus; On Diagnosis from Dreams)
Claudius Galenus of Pergamum (c. 130 – c. 200), better known as Galen, was a Greek physician and a writer on medicine and philosophy. His theories dominated European medicine for well over a millennium.
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Galen of Pergamum (AD 129 – 199/217) was a prominent Roman physician and philosopher from Greece. He was an important medical researcher during the time of the Roman Empire. His theories influenced Western medical science for over a thousand years.
He came up with the 'Theory of Opposites' based upon Hippocrates' 'Four Humours' theory, which helped develop medicine in the Roman and again in the Middle Ages to include more natural based medicine (rather that just religious medicine). Also, in the Middle Ages, after the Dark Ages, Galen was upheld as the 'Official Doctor of the Church', due to a 'Creator' being mentioned in his books.
Some of his observations in his books were wrong, as he could only dissect animals and compare them to humans, so later on in the Renaissance period people such as Vesalius challenged his theories and observations with his own books.
Galen was vital in the beginnings of natural medicine, and is often known as the 'Father of Anatomy'.rue:Ґален