Early descriptions of the lungs emp

Early descriptions of the lungs emphasized their importance as cooling agents that maintained the balance of the human body by counteracting the hot temperament of the heart. They also understood that breathing occurred in the lungs, in essence, that the lungs acted like a pair of bellows firing and cooling a furnace. Galen's description, for example, of the lung emphasized that it "has all the properties which make for easy evacuation; for it is very soft and warm and is kept in constant motion." He further specified the function of the lungs in relationship to the movement of the blood as follows: "Blood passing through the lungs absorbed from the inhaled air, the quality of heat, which it then carried into the left heart." From such passages, we can see that the idea of respiration was primarily influenced by humoral theories of the body.

Medieval medical practitioners continued to be fascinated by the unusual anatomy of the lung. They noted its unusual moisture, for example, as a feature of its complexion. In the early eleventh century, the Islamic medical philosopher Avicenna wrote in his Canon of Medicine: "In the case of the lung the moisture is not inherent in its nature but is derived from the nourishment which comes to it. The lung is fed by a very 'hot' blood, because there is much bilious humor in the blood going to the lung. A great excess of moisture accumulates in the lung from the gaseous products of the whole body as well as from the materials which flow down to it from the "head." By the late twelfth century, Master Nicolaus summarized well the qualities of the lung: it was built to "withstand the warmth of the heart," its soft, spongy tissue facilitated motion, it was hollow in order to retain air "for cooling the heart and renewing the vital spirits," and it exhibited a twofold motion that further facilitated its role as "the flail of the heart."

Medieval and Renaissance physicians understood the connection between the lungs and respiration and between life and breath. They associated life with a vitality that coursed throughout the body, a Galenic pneuma. But they had no specific understanding of the role that oxygen played, since they had no chemical understanding of air which was one of the four basic "elements" of nature in traditional Greek science. Despite this fact, they did understand that the lung discharged wastes. Leonardo da Vinci describes the process as follows in his unpublished notebooks of the late fifteenth century: "From the heart, impurities or 'sooty vapors' are carried back to the lung by way of the pulmonary artery, to be exhaled to the outer air." His contemporary, the physician Alessandro Benedetti concurred, writing in 1497, "The lung changes the breath, as the liver changes the chyle, into food for the vital spirit.