HARVEY AND EXPERIMENTAL OBSERVATION 47 



and Michael Foster are among the most judicial; that of 

 Foster, indeed, inasmuch as it contains ample quotations 

 from the original sources, is the most nearly complete and 

 satisfactory. The discussion is too long to enter into fully 

 here, but a brief outline is necessary to understand what 

 he accomplished, and to put his discovery in the proper 



light. 



To say that he first discovered — or, more properly, 

 demonstrated — the circulation of the blood carries the im- 

 pression that he knew of the existence of capillaries connect- 

 ing the arteries and the veins, and had ocular proof of the 

 circulation through these connecting vessels. But he did not 

 actually see the blood moving from veins to arteries, and he 

 knew not of the capillaries. He understood clearly from his 

 observations and experiments that all the blood passes from 

 veins to arteries and moves in "a kind of circle"; still, he 

 thought that it filters through the tissues in getting from one 

 kind of vessel to the other. It was reserved for Malpighi, 

 in 1661, and Leeuwenhoek, in 1669, to see, with the aid of 

 lenses, the movement of the blood through the capillaries 

 in the transparent parts of animal tissues. (See under 

 Leeuwenhoek, p. 84.) 



The demonstration by Harvey of the movement of the 

 blood in a circuit was a matter of cogent reasoning, based on 

 experiments with ligatures, on the exposure of the heart in 

 animals and the analysis of its movements. It has been com- 

 monlv maintained (as bv Whewell) that he deduced the cir- 

 culation from observations of the valves in the veins, but this 

 is not at all the case. The central point of Harvey's reason- 

 ing is that the quantity of blood which leaves the left cavity 

 of the heart in a given space of time makes necessary its 

 return to the heart, since in a half-hour (or less) the heart, 

 by successive pulsations, throws into the great artery more 

 than the total quantity of blood in the body. Huxley points 



