This principle must never be forgotten in our
experiments : otherwise we shall imagine we see
the effects of irritability in vessels, when, in fact,
we only see the phenomena resulting from tension;
or we shall imagine we see the loss of irritability,
when it is but the effect of obstruction,
and the power of tension yielding to those of
the circulation. The effect of a ligature applied
to a vein, and the effect of those substances
applied to the web, which induce stagnation
by altering the state of the interior membrane
of the vessels, are examples of the latter kind.
In both cases, the tension of the vessels or
integuments yields to the forces which propel
the blood, and the vessels appear enlarged.
Incisions made through the membranes of the
web or mesentery, always assume a circular or
oval shape, by the operation of this tension and
elasticity of these membranes, of which therefore
there can be no donbt.
This tension of the integuments is, I think,
the source of many of the phenomena of the
minute and capillary circulation.
INTRODUCTION.
OF THE PRINCIPLES OF INVESTIGATION IN
PHYSIOLOGY.
The sources of our knowledge in physiology, as
in all natural science, are observation and experiment
: the former consists in a sustained and
watchful attention to events which pass under our
eye in the ordinary course of nature ; the latter,
in devices for placing natural objects in new and
unusual circumstances or situations.
Unhappily for the physiologist, the subjects
of the principal department of his science, that
of animal physiology, are sentient beings; and
every experiment, every new or unusual situation
of such a being, is necessarily attended by pain
or suffering of a bodily or mental kind. Investigations
in this science should, therefore, being
B