WILLIAM HARVEY, M. D. 63 



when it comes to the circulation of the blood, the demonstra- 

 tion is logical, not ocular. The absence of magnifying instru- 

 ments of sufficient strength at the time made it impossible to 

 observe either the circulation of the blood or the continuity of 

 the cardiovascular system. It is not implied here, however, that 

 the ocular, even if possible, could approach or match the certi- 

 tude of the logical demonstration.^^ 



Circulation, as such, is not mentioned in the body of the 

 work until Chapter 8, where it is introduced in the form of a 

 short review of the argument developed subsequently. Since 

 the conclusion that the circulation of the blood is the end result 

 of a long reasoning process, the chief function of Harvey's pre- 

 ceding chapters is to contribute premises which are ti*ue, 

 primary, immediate, better known than, prior to, and the cause 

 of the conclusions which follow from them.** In other words, 

 it is necessary to establish the motion, pulse, and action of 

 the heart and arteries, and the relationship of the lungs to the 

 heart and the blood to the lungs first. This calls for the most 

 exacting type of sense observations, their verification by col- 

 lated findings, and care in the inferences drawn from them. It 

 is through such knowledge that Harvey is in a position to ask 

 questions leading to the initial idea and final demonstration 

 that the blood circulates. 



The first part of Harvey's treatise establishes, contrary to 

 the beliefs at the time, that the heart and the arteries in the 

 living animal always contain blood: that the proper motion of 

 the heart is contraction, not expansion; that its action is pump- 

 like, not bellow-like, and that it forcibly expels blood in one 

 direction; that contraction, not expansion — systole, not diastole 

 — corresponds to the pulse on the chest wall; that the arterial 



** It should not be forgotten that the observations of Swammerdam of the per- 

 fectly formed butterfly in the cocoon in 1669, and those of Leeuwenhoek of the com- 

 plete outline of both maternal and paternal individuals in the microscopic sperma- 

 tozoa in 1677, led to the complete replacement of Harvey's theory of epigenesis by 

 the preformation theory, which lent itself to a mechanical explanation of nature, 

 and which was to dominate biological thinking through the first half of the eight- 

 eenth century. 



"Aristotle, Posterior Analytics, Bk. 1, ch. 1, 71 b 16-22. 



