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- 
monly maintained (as by Whewell) that he deduged 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 
