No part of the brain or medullas appears to be
immediately necessary to the action of the heart.
But the arterialization of the blood seems essential
to secretion and nutrition. This change goes
on in the chick without the act of respiration;
and in the frog it is partially effected by the skin.
It is possible that, in this latter case, we have but
to support the respiration, to continue life, with
the secretions, independently of the brain and
spinal marrow.
It is a remarkable fact, observed in the web
of the frog, that the minute nerves pursue a
course close to the minute arteries. The terminal
nerves and arteries may ultimately join and form
the immediate secretory organ.
In every point of view the batrachia seem to
offer the objects most likely to enable us to
explore the connection of the large nervous
masses with the heart, the minute and capillary
circulation, and the secretory organs and
processes.
In conclusion, I would observe, that there is
not an experiment detailed in the following pages,
which has not been repeated in the presence of
some of the first physiologists of this country.
Before 1 conclude this short preface, I must
introduce two postscripts to Chapter II.
P. S. 1. In the first place, I would observe that
some further deductions may probably be drawn
from the experiment of applying a ligature round
the large vessels connected with the heart (described
p. 78), beyond that of the irritability of
the arteries.
After the power of the heart is thus excluded,
what do we observe ? A peculiar slow oscillatory
movement of the blood in the arteries, whilst that
in the capillaries and veins is motionless.
If the capillaries and veins have the same
power of irritability as the arteries, why do we
not observe the same phenomenon in them as in
the arteries ?
If the capillaries possess a muscular power, why
is the blood all at once motionless in them, when
the power of the heart is excluded ? Must it
not rather be inferred, that, as the blood contained
in these vessels is without motion, the
vessels themselves are without automatic power ?
The same argument applies to the minute veins.