arize--until the seventeenth century there were believed to
be two closed systems in the circulation, (1) the natural, containing
venous blood, had its origin in the liver from which, as from a
fountain, the blood continually ebbed and flowed for the nourishment of
the body; (2) the vital, containing another blood and the spirits, ebbed
and flowed from the heart, distributing heat and life to all parts. Like
a bellows the lungs fanned and cooled this vital blood. Here and there
we find glimmering conceptions of a communication between these systems,
but practically all teachers believed that the only one of importance
was through small pores in the wall separating the two sides of the
heart. Observation--merely looking at and thinking about things--had
done all that was possible, and further progress had to await the
introduction of a new method, viz., experiment. Galen, it is true, had
used this means to show that the arteries of the body contained blood
and not air. The day had come when men were no longer content with
accurate description and with finely spun theories and dreams. It was
reserved for the immortal Harvey to put into practice the experimental
method by which he demonstrated conclusively that the blood moved in a
circle. The "De Motu Cordis" marks the final break of the modern spirit
with the old traditions. It took long for men to realize the value of
this "inventum mirabile" used so effectively by the Alexandrians--by
Galen--indeed, its full value has only been appreciated within the past
century. Let me quote a paragraph from my Harveian Oration.(33) "To the
age of the hearer, in which men had heard and heard only, had succeeded
the age of the eye in which men had seen and had been content only
to see. But at last came the age of the hand--the thinking, devising,
planning hand, the hand as an instrument of the mind, now re-introduced
into the world in a modest little monograph from which we may date the
beginning of experimental medicine."

(33) Osler: An Alabama Student, etc., pp. 329-330.

Harvey caught the experimental spirit in Italy, with brain, eye and hand
as his only aids, but now an era opened in which medicine was to derive
an enormous impetus from the discovery of instruments of precision. "The
new period in the development of the natural sciences, which reached
its height in the work of such men as Galileo, Gilbert and Kepler, is
chiefly characterized by the invention of very importa