Never satisﬁed with purely anatomical description, Galen constantly struggled to ﬁnd ways of proceeding from structure to function, from pure anatomy to experimental physiology. It is rare to encounter a problem in what might be called classical physiology that Galen did not attempt to cope with either by experiment or speculation. By extending medical research from anatomy to physiology, Galen estab- lished the foundations of a program that would transform the Hippoc- ratic art of medicine into the science of medicine.
In formulating his physiological principles, Galen was sometimes misled by preconceived and erroneous ideas and hindered by the techni- cal difﬁculties inherent in such investigations. Given the magnitude of his self-imposed task, and the voluminous and prolix nature of his writings, the totality of his work has been more honored than under- stood. His errors, which ultimately stimulated revolutions in anatomy and physiology in the sixteenth and seventeenth centuries, tend to be overemphasized. It is important, therefore, to balance the merits and the defects in the powerful Galenic synthesis that was to satisfy the needs of scholars and physicians for hundreds of years.
Galen’s system of physiology encompassed concepts of blood formation, respiration, the heartbeat, the arterial pulse, digestion, nerve function, embryology, growth, nutrition, and assimilation. Galenic physiology rested on the Platonic doctrine of a threefold division of the soul. This provided a means of dividing vital functions into pro- cesses governed by vegetative, animal, and rational ‘‘souls’’ or ‘‘spirits.’’ Within the human body, pneuma (air), which was the breath of the cos- mos, was subject to modiﬁcations brought about by the innate faculties of the three principle organs—the liver, heart, and brain—and distrib- uted by three types of vessels—veins, arteries, and nerves. The Galenic system is complex and often obscure. Moreover, it is difﬁcult and per- haps counterproductive to attempt absolute distinctions between what Galen actually said and the way in which his doctrines were understood and handed down by later interpreters. In any event, Galen sometimes said different things in different texts and, since not all of his writings have survived, it is possible that interpretations made by particular com- mentators could have been based on manuscripts that have been lost.
In essence, according to Galen’s system, pneuma was modiﬁed by the liver so that it became the nutritive soul or natural spirits that sup- ported the vegetative functions of growth and nutrition; this nutritive soul was distributed by the veins. The heart and arteries were respon- sible for the maintenance and distribution of innate heat and pneuma or vital spirits to warm and vivify the parts of the body. The third adap- tation, which occurred in the brain, produced the animal spirits required
for sensation and muscular movement; the animal spirits were distrib- uted through the nerves. Sometimes Galen’s arguments concerning particular problems suggest reservations about the functions of the spirits, but he was certain that animal life is only possible because of the existence of pneuma within the body.
Because of the central role theories of the motion of the heart and blood have played in the history of Western medical science, Galen’s views on this topic have been the subject of considerable attention and controversy. Part of the difﬁculty in reconstructing a simpliﬁed ver- sion of Galen’s concept of this problem resides in the fact that respi- ration and the movement of the blood are so intimately linked in Galen’s system that it is difﬁcult to unravel the threads of each problem, or consider them apart from his doctrines concerning the elaboration and distribution of pneuma and spirits. Respiration, which was thought to be involved in cooling the excess heat of the heart, was obviously necessary for life. Therefore, vital spirit is necessarily associated with the organs of respiration, which in Galen’s system included the heart and arteries as well as the lungs. If the natural spirit exists, Galen thought it would be contained in the liver and the veins. Attempting to simplify Galen’s prolix arguments, his followers often transformed tentative ‘‘if there are’’ hypotheses into dogmatic ‘‘there are’’ certainties.
In Galen’s physiological scheme, blood was continuously synthe- sized from ingested foods. The useful part of the food was transported as chyle from the intestines via the portal vein to the liver, where, by virtue of the innate faculty of the liver, it was transformed into dark venous blood. Tissues could then suck up the nutriments they needed from the blood by virtue of their faculty for speciﬁc selection. The use- less part of the food was converted into black bile by the spleen. Even Galen could not come to grips with the precise means by which such transformations—all the complex phenomena now subsumed by the term metabolism—might be effected.
Like Erasistratus, Galen assumed that there must be connections between the veins (which arose from the liver) and the arteries (which arose from the heart) because bleeding from any vessel could drain the whole system. But Galen ingeniously refuted the idea that, under normal conditions, the arteries contain only air. His arguments and experimental proof were set forth in a brief work entitled Whether Blood Is Contained in the Arteries in Nature. If the artery of a living animal is exposed and tied it off at two points, the section of the vessel between the ligatures is full of blood. Moreover, when the chest of a living animal is opened, blood is found in the left ventricle of the heart. According to Galen’s scheme, the arterial pulse was generated by the heart. During the diastole of the heart, the dilation of the arteries drew in air through the pores in the skin and blood from the veins. Thus, the arteries served the function of nourishing the innate heat throughout the body. This concept could be demonstrated by tying a ligature around a limb so that it was tight enough to cut off the arterial pulse. Below the ligature, the limb would become cold and pale, because the arteries were no longer able to supply the innate heat.
Although Galen gave a good description of the heart, its chambers and valves, his preconceived concepts led to ambiguities, misinterpre- tations, and even misrepresentations of anatomical observations. For Galen’s system to work, blood had to pass from the right ventricle to the left ventricle. Therefore, he assumed that blood in the right side of the heart could follow various paths. Some of the blood carried impu- rities, or ‘‘sooty vapors,’’ for discharge by the lungs via the artery-like vein (pulmonary artery). Blood could also pass from the right side to the left side of the heart by means of pores in the septum. The pores themselves were not visible, but Galen assumed that the pits found in the septum were the mouths of the pores. The idea that the blood tended to ebb and ﬂow like the tide was long associated with Galenic physi- ology although this seems to be a misinterpretation of Galen’s generally vague statements about the movement of blood within the vessels. How- ever, the system seems to depend on an obscure two-way movement of blood through certain vessels.
After appropriate ‘‘digestion’’ in the lungs, inhaled air was brought to the heart by the pulmonary vein. The modiﬁed air was further acted on in the heart and transported to other parts of the body by the arteries. Arterial blood was especially ﬁne and vaporous so that it could nourish the vital spirit. Further reﬁnement was accomplished in the arteries that formed the rete mirabile—a network of vessels found at the base of the brain of oxen and other animals, but not in humans. The transformation of arterial blood into animal spirits in the brain and their distribution via the nerves completed the threefold system of spirits.
Clearly, the concept of blood circulation is incompatible with a scheme in which blood is constantly synthesized by the liver to be assimilated or consumed as it ebbs and ﬂows in the blood vessels. Of course the nature of the Galenic system is so complex that ‘‘clearly’’ is hardly an appropriate word to use in a brief description of it. Rather than throw any further obscurity on the subject, let us consider Galen’s ideas about the treatment of diseases.