In the earlier years of the sixteenth century, when anatomy and physiology were beginning to make progress, the savants of that day, hampered as they were by grave misconceptions and by the lack of any understanding of chemical phenomena, could not take advantage, naturally, of the suggestion that as wood burns or oxidizes in the air with liberation of heat, so might the food substances, absorbed by the body, undergo oxidation in the tissues and thus give rise to animal heat. Such suggestions were at that time as a closed book, and so we find Vesalius, in 1543, teaching the Galenic doctrines in physiology then prevalent. The conception of heat production, as it existed at that time, may be inferred from the following quotation:[15] “The parts of the food absorbed from the alimentary canal are carried by the portal blood to the liver, and by the influence of that great organ are converted into blood. The blood thus enriched by the food is by the same great organ endued with the nutritive properties summed up in the phrase ‘natural spirits.’ But blood thus endowed with natural spirits is still crude blood, unfitted for the higher purposes of the blood in the body. Carried from the liver by the vena cava to the right side of the heart, some of it passes from the right ventricle through innumerable invisible pores in the septum to the left ventricle. As the heart expands it draws from the lungs through the vein-like artery air into the left ventricle. And in that left cavity, the blood which has come through the septum is mixed with the air thus drawn in, and by the help of that heat, which is innate in the heart, which was placed there as the source of the heat of the body by God in the beginning of life, and which remains there until death, is imbued with further qualities, is laden with ‘vital spirits,’ and so fitted for its higher duties. The air thus drawn into the left heart by the pulmonary vein, at the same time tempers the innate heat of the heart and prevents it from becoming excessive.” In other words, heat was considered as a divine gift, and as can readily be seen, there was an utter lack of appreciation of the use of air in breathing. Even van Helmont, who lived in 1577–1644, and was in a sense an alchemist, still gave credence to the spirits, viz., that the food absorbed from the stomach and intestine is in the liver endued with natural spirits, while in the heart the natural spirits are converted into vital spirits, and in the brain the vital spirits are transformed into animal spirits.[16] Later, Malpighi discovered the true structure of the lungs, and Borelli, in 1680, exposed the erroneous views then prevalent regarding the purpose of breathing. It is not true, says Borelli, that the use of breathing is to cool the excessive heat of the heart or to ventilate the vital flame, but we must believe that this great machinery of the lungs, with their accompanying blood vessels, is for some grand purpose. In a long and