in which
this point could be settled.

Fig.2,--The Course of the Blood according to Galen (A.D. 170).

And hence we are indebted, for the correction of the error of
Erasistratus, to one of the greatest experimenters of ancient or modern
times, Claudius Galenus, who lived in the second century after Christ. I
say it was to this man more than any one else, because he knew that the
only way of solving physiological problems was to examine into the facts
in the living animal. And because Galen was a skilful anatomist, and
a skilful experimenter, he was able to show in what particulars
Erasistratus had erred, and to build up a system of thought upon this
subject which was not improved upon for fully 1,300 years. I have
endeavoured, in Fig. 2, to make clear to you exactly what it was he
tried to establish. You will observe that this diagram is practically
the same as that given in Fig. 1, only simplified. The same facts may
be looked upon by different people from different points of view. Galen
looked upon these facts from a very different point of view from that
which we ourselves occupy; but, so far as the facts are concerned, they
were the same for him as for us. Well then, the first thing that Galen
did was to make out experimentally that, during life, the arteries are
not full of air, but that they are full of blood. And he describes a
great variety of experiments which he made upon living animals with the
view of proving this point, which he did prove effectually and for all
time; and that you will observe was the only way of settling the matter.
Furthermore, he demonstrated that the cavities of the left side of the
heart--what we now call the left auricle and the left ventricle--are,
like the arteries, full of blood during life, and that that blood was of
the scarlet kind--arterialised, or as he called it "pneumatised," blood.
It was known before, that the pulmonary artery, the right ventricle,
and the veins, contain the darker kind of blood, which was thence called
venous. Having proved that the whole of the left side of the heart,
during life, is full of scarlet arterial blood, Galen's next point
was to inquire into the mode of communication between the arteries
and veins. It was known before his time that both arteries and veins
branched out. Galen maintained, though he could not prove the fact, that
the ultimate branches of the arteries and veins communicated together
somehow or other, by what he called 'anastomo