Juxtaposition of Actuarius' versus Galen's ideas on renal physiology: the impact of 12 centuries.

Although some elements of renal physiology can be traced to scattered references in Greek medical writings, mainly in Hippocrates' and Erasistratus' works, it was Galen who made the first breakthrough observations regarding the function of the kidneys. He often wrote his observations not as a diatribe, but as a confrontation with other physicians, mainly of the Erasistratian School. He outlined the great importance of the disproportionally large blood supply of the kidneys, an over-proportion not observed in any other organ, rightly arguing that this is a teleological procedure to achieve satisfactory body clearance. He challenged the dominant idea that blood purification and hence the formation of urine took place in the large branches of the renal vessels. He strongly argued that the special structure of the kidney tissue was responsible for their cathartic ability. Although he had no idea of the histology of the kidney, he made some beautiful similitudes to explain the filtering capacity of the renal tissue. From Galen onwards, very few changes and additions can be traced in Greek and thereafter Byzantine medical texts, regarding renal physiology. It is Johannes Actuarius who, 12 centuries later, writes an extensive treatise on urine and presents his point of view on renal physiology, mainly as conclusions from the pathological findings of uroscopy. In the present paper, we compare the 2 physicians' approaches on the function of the kidneys, which can be attributed to the accumulation of medical knowledge over the 12 centuries that separate them.