othesis that this
is their normal condition, and that during life they are filled with
air. And it will be observed that it is not improbable that
Erasistratus' discovery of the valves of the heart and of their
mechanical action strengthened him in this view. For, as the arteria
venosa branches out in the lungs, what more likely than that its
ultimate ramifications absorb the air which is inspired; and that this
air, passing into the left ventricle, is then pumped all over the body
through the aorta, in order to supply the vivifying principle which
evidently resides in the air; or, it may be, of cooling the too great
heat of the blood? How easy to explain the elastic bounding feel of a
pulsating artery by the hypothesis that it is full of air! Had
Erasistratus only been acquainted with the structure of insects, the
analogy of their tracheal system would have been a tower of strength to
him. There was no _prima-facie_ absurdity in his hypothesis--and
experiment was the sole means of demonstrating its truth or falsity.

More than four hundred years elapsed before the theory of the motion of
the blood returned once more to the strait road which leads truthward;
and it was brought back by the only possible method, that of experiment.
A man of extraordinary genius, Claudius Galenus, of Pergamus, was
trained to anatomical and physiological investigation in the great
schools of Alexandria, and spent a long life in incessant research,
teaching, and medical practice. More than one hundred fifty treatises
from his pen, on philosophical, literary, scientific, and practical
topics, are extant; and there is reason to believe that they constitute
not more than a third of his works. No former anatomist had reached his
excellence, while he may be regarded as the founder of experimental
physiology. And it is precisely because he was a master of the
experimental method that he was able to learn more about the motions of
the heart and of the blood than any of his predecessors, and to leave to
posterity a legacy of knowledge which was not substantially increased
for more than thirteen hundred years.

The conceptions of the structures of the heart and vessels, of their
actions, and of the motion of the blood in them, which Galen
entertained, are not stated in a complete shape in any one of his
numerous works. But a careful collation of the various passages in which
these conceptions are expressed leaves no doubt upon my mind that
Galen's