And yet with all this the fact remains that Harvey never really knew, from the nature of the case could not know, how the blood passed from the arteries to the veins--how, in other words, an essential part of the circulation was actually accomplished. The blood passes from the arteries to the veins through minute microscopic tubes termed capillaries. In Harvey’s day the microscope was not sufficiently powerful to reveal such fine structures to human vision, and he was therefore necessarily ignorant of their existence. Looked at from this point of view, the discovery affords a very good example of what has been aptly termed the scientific use of the imagination. Although, with his imperfect microscope, it was impossible for him to know how the blood actually passed from the arteries to the veins, yet as the result of his observations and experiments he was able to infer and to state the grounds for his inference in clear, forcible, and most convincing language, that the blood must circulate, and circulate in one direction only, viz. from the heart into the arteries, thence to the veins, by which it was brought back to the heart again. His imagination was thus enabled to bridge over the gulf between the arteries and veins which his eyes, with the imperfect instrument then alone at his disposal, were quite unable to cross. It was not until four years after Harvey’s death that the microscope had been sufficiently improved to enable an Italian anatomist named Malpighi,[10] in the year 1661, to actually observe the capillaries uniting by their networks arteries and veins. [10] De Pulmonibus, Observationes Anatomici, 1663. The work was published at Frankfort doubtless that it might be more easily disseminated over the Continent. It made a sensation among the learned of all countries. Its conclusions were opposed by the older physicians; but by the younger scientific men it was by no means received with disfavour. Amongst the latter was the philosopher Descartes, whose name was then a power in Europe. The philosophical, yet keenly practical mind of Descartes grasped the discovery with avidity and supported it with ardour. In his celebrated “Discours de la Méthode,”[11] he refers to the discovery of “an English physician,” and describes with enthusiasm the anatomy and use of the heart. Although we have no certain information on the point it is quite possible that Descartes may have known Harvey, for in the year 1631 he is said to have paid a visit to England; and in his second reply to Riolan Harvey refers to “the ingenious and acute Descartes,” and says the honourable mention of his name demands his acknowledgments. Thus the discovery became widely known and largely adopted. [11] First published at Leyden in 4to, 1637.