xvi Editor's Introduction 



on it. This assuredly is a fact almost unique in the 

 history of science. 



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 im- 

 possible 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 anato- 

 mist named Malpighi, 1 in the year i66r, to actually 

 observe the capillaries uniting by their networks arteries 

 and veins. 



The work was published at Frankfort doubtless that 

 it might be more easily disseminated over the Con- 

 tinent. It made a sensation among the learned of all 

 countries. Its conclusions were opposed by the older 



1 De Pulmonibus, Observationes Anatomici, 1663. 



