46 CIRCULATION OF THE BLOOD 



through the lungs into the aorta, as by two clacks of a water 

 bellows to raise water. Moreover, on account of the action 

 of a bandage on the vessels of the arm [see p. 58 below] 

 he is of the opinion that there is a transit of blood from 

 the arteries to the veins. It is thus demonstrated that a 

 perpetual motion of the blood in a circle is brought about by 

 the beat of the heart. What shall we say ? Is this for 

 the purpose of nutrition ? Or is it for the better pre- 

 servation of the blood and of the members by imparting 

 heat to them, the blood by turns losing heat as it warms 

 the members, and gaining heat from the heart ? ' 



The three sentences, " the blood is constantly passed 

 through the lungs into the aorta as by two clacks of a 

 water bellows to raise water," ' there is a transit of blood 

 from the arteries to the veins," and " a perpetual motion 

 of the blood in a circle is brought about by the beat of 

 the heart," conclusively prove that Harvey had already 

 attained to an understanding of the essential points in 

 the circulation of the blood. 



One phrase requires a little further explanation. It 

 is the curious expression in which the action of the heart 

 in driving blood through the lungs is compared to " two 

 clacks of a water bellows to raise water." A clack in the 

 English of Harvey's day was a form of valve used on 

 pumps or " water bellows." Such a valve or " clack ' 

 was opened by the upward movement of the water 

 produced by suction, and closed again by the backward 

 pressure of the weight of water. Thus a writer contem- 

 porary with Harvey tells us that ' ' a clacke is a piece of 

 Leather nayled ouer any hole hauing a peece of Lead 

 to make it lie close, so that ayre or water in any vessell 

 may thereby bee kept from going out." l The two 



1 John Bate, Mysteries and Arts in foure seuerall Parts, London, 

 1635. The quotation we have given is fronl Part I. Of Water 

 Works. 



