distributing it through the arteries. But, as we know that he was
aware of the fact that the arteries also contained blood, he must
have believed that these vessels contained a mixture of the two.

Modern anatomists know that the heart is divided into two
approximately equal parts by an impermeable septum of tough
fibres. Yet, Galen, who dissected the hearts of a vast number of
the lower animals according to his own account, maintained that
this septum was permeable, and that the air, entering one side of
the heart from the lungs, passed through it into the opposite
side and was then transferred to the arteries.

He was equally at fault, although perhaps more excusably so, in
his explanation of the action of the nerves. He had rightly
pointed out that nerves were merely connections between the brain
and spinal-cord and distant muscles and organs, and had
recognized that there were two kinds of nerves, but his
explanation of the action of these nerves was that "nervous
spirits" were carried to the cavities of the brain by
blood-vessels, and from there transmitted through the body along
the nerve-trunks.

In the human skull, overlying the nasal cavity, there are two
thin plates of bone perforated with numerous small apertures.
These apertures allow the passage of numerous nerve-filaments
which extend from a group of cells in the brain to the delicate
membranes in the nasal cavity. These perforations in the bone,
therefore, are simply to allow the passage of the nerves. But
Galen gave a very different explanation. He believed that impure
"animal spirits" were carried to the cavities of the brain by the
arteries in the neck and from there were sifted out through these
perforated bones, and so expelled from the body.

He had observed that the skin played an important part in cooling
the body, but he seems to have believed that the heart was
equally active in overheating it. The skin, therefore, absorbed
air for the purpose of "cooling the heart," and this cooling
process was aided by the brain, whose secretions aided also in
the cooling process. The heart itself was the seat of courage;
the brain the seat of the rational soul; and the liver the seat
of love.

The greatness of Galen's teachings lay in his knowledge of
anatomy of the organs; his weakness was in his interpretations of
their functions. Unfortunately, succeeding generations of
physicians for something like a thousand years rejected the
former but clung to the latter, so that the advances he had made
were completely overshadowed by the mistakes of his teachings.



XI. A RETROSPECTIVE GLANCE AT CLASSICAL SCIENCE

It is a favorite tenet of the modern historian that history is a
continuous stream. The contention has fullest warrant. Sharp
lines of demarcation are an evidence of man's analytical
propensity rather than the work of nature. Nevertheless it would
be absurd to deny that the stream of history presents an
ever-varying current. There are times when it seems to rush
rapidly on; times when it spreads out into a broad--seemingly
static--current; times when its catastrophic changes remind us of
nothing but a gigantic cataract. Rapids and whirlpools, broad
estuaries and tumultuous cataracts are indeed part of the same
stream, but they are parts that vary one from another in their
salient features in such a way as to force the mind to classify
them as things apart and give them individual names.

So it is with the stream of history; however strongly we insist
on its continuity we are none the less forced to recognize its
periodicity. It may not be desirable to fix on specific dates as
turning-points to the extent that our predecessors were wont to
do. We may not, for example, be disposed to admit that the Roman
Empire came to any such cataclysmic finish as the year 476 A.D.,
when cited in connection with the overthrow of the last Roman
Empire of the West, might seem to indicate. But, on the other
hand, no student of the period can fail to realize that a great
change came over the aspect of the historical stream towards the
close of the Roman epoch.

The span from Thales to Galen has compassed about eight hundred
years--let us say thirty generations. Throughout this period
there is scarcely a generation that has not produced great
scientific thinkers--men who have put their mark upon the
progress of civilization; but we shall see, as we look forward
for a corresponding period, that the ensuing thirty generations
produced scarcely a single scientific thinker of the first rank.
Eight hundred years of intellectual activity --thirty generations
of greatness; then eight hundred years of stasis--thirty
generations of mediocrity; such seems to be the record as viewed
in perspective. Doubtless it seemed far different to the
contemporary observer; it is only in reasonable perspective that
any scene can be viewed fairly. But for us, looking back without
prejudice across the stage of years, it seems indisputable that a
great epoch came to a close at about the time when the barbarian
nations of Europe began to sweep down into Greece and Italy. We
are forced to feel that we have reached the limits of progress of
what historians are pleased to call the ancient world. For about
eight hundred years Greek thought has been dominant, but in the
ensuing period it is to play a quite subordinate part, except in
so far as it influences the thought of an alien race. As we leave


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