know, is the first time that quantitative
considerations are taken into account in the discussion of a
physiological problem. But one of the most striking differences between
ancient and modern physiological science, and one of the chief reasons
of the rapid progress of physiology in the last half-century, lies in
the introduction of exact quantitative determinations into physiological
experimentation and observation. The moderns use means of accurate
measurement which their forefathers neither possessed nor could
conceive, inasmuch as they are products of mechanical skill of the last
hundred years, and of the advance of branches of science which hardly
existed, even in germ, in the seventeenth century.

Having attained to a knowledge of the circulation of the blood, and of
the conditions on which its motion depends, Harvey had a ready deductive
solution for problems which had puzzled the older physiologists. Thus
the true significance of the valves in the veins became at once
apparent. Of no importance while the blood is flowing in its normal
course toward the heart, they at once oppose any accidental reversal of
its current which may arise from the pressure of adjacent muscles or the
like. And in like manner the swelling of the veins on the farther side
of the ligature, which so much troubled Caesalpinus, became at once
intelligible as the natural result of the damming up of the returning
current.

In addition to the great positive results which are contained in the
treatise which Harvey modestly calls an _Exercise_ and which is, in
truth, not so long as many a pamphlet about some wholly insignificant
affair, its pages are characterized by such precision and simplicity of
statement, such force of reasoning, and such a clear comprehension of
the methods of inquiry and of the logic of physical science, that it
holds a unique rank among physiological monographs. Under this aspect, I
think I may fairly say that it has rarely been equalled and never
surpassed.

Such being the state of knowledge among his contemporaries, and such the
immense progress effected by Harvey, it is not wonderful that the
publication of the _Exercitatio_ produced a profound sensation. And the
best indirect evidence of the originality of its author, and of the
revolutionary character of his views, is to be found in the multiplicity
and the virulence of the attacks to which they were at once subjected.

Riolan, of Paris, had the greatest reputa