tablishment of the
doctrine of the _Circulation of the Blood_.

In this case, _simple observation_ yields us a knowledge of the
existence of the blood from some accidental haemorrhage, we will say: we
may even grant that it informs us of the localization of this blood in
particular vessels, the heart, &c., from some accidental cut or the
like. It teaches also the existence of a pulse in various parts of the
body, and acquaints us with the structure of the heart and vessels.

Here, however, _simple observation_ stops, and we must have recourse to
_experiment_.

You tie a vein, and you find that the blood accumulates on the side of
the ligature opposite the heart. You tie an artery, and you find that
the blood accumulates on the side near the heart. Open the chest, and
you see the heart contracting with great force. Make openings into its
principal cavities, and you will find that all the blood flows out, and
no more pressure is exerted on either side of the arterial or venous
ligature.

Now all these facts, taken together, constitute the evidence that the
blood is propelled by the heart through the arteries, and returns by the
veins--that, in short, the blood circulates.

Suppose our experiments and observations have been made on horses, then
we group and ticket them into a general proposition, thus:--_all horses
have a circulation of their blood_.

Henceforward a horse is a sort of indication or label, telling us where
we shall find a peculiar series of phaenomena called the circulation of
the blood.

Here is our _general proposition_ then.

How and when are we justified in making our next step--a _deduction_
from it?

Suppose our physiologist, whose experience is limited to horses, meets
with a zebra for the first time,--will he suppose that this
generalization holds good for zebras also?

That depends very much on his turn of mind. But we will suppose him to
be a bold man. He will say, "The zebra is certainly not a horse, but it
is very like one,--so like, that it must be the 'ticket' or mark of a
blood-circulation also; and, I conclude that the zebra has a
circulation."

That is a deduction, a very fair deduction, but by no means to be
considered scientifically secure. This last quality in fact can only be
given by _verification_--that is, by making a zebra the subject of all
the experiments performed on the horse. Of course, in the present case,
the _deduction_ would be _confirmed_ by this process of