traction, and the oscillating globules in the adjacent
branch, are distinctly seen by the aid of the
microscope. The drawing was taken from an
injected preparation of this artery.
The frog or toad exposed, as has been described,
p. 64, to water of 120°. Fahr. presents
all these phenomena most distinctly.
In this fact we have the most, and, I may
add, the only, indubitable proof of a contractile
action in an artery.
It is interesting to contrast the appearance of
automatic contraction in this artery, with that
of dilatation, the effect of the impulse of blood
sent from the heart, as seen in the principal
pulmonary artery in the batrachia. Whilst the
former becomes straighter and paler, the latter
is rendered still more tortuous, and is still more
distended with blood, Mr. Hunter justly observes,
“ arteries during their diastole, which arises
from an increased quantity of blood being thrown
into them, increase much more in length than
width, being thrown into a serpentine course;
therefore, instead of the term diastole, it should
rather be called the elongated state.” 1
1 On the Blood, &c. Ed. 1791, p. 175.
The arteries are, indeed, a second heart in
an elongated form. Their function appears to be
so perfectly performed in health, that all visible
pulsation from their action is lost at their extreme
branches. The blood is carried along at last by
such gentle undulations, caught in their full force
from the heart, but softened first by the elas-
ticity, and then by the contraction of successive
portions of the arteries, that its flow seems to
become uniform. It is as the powers of the system
languish, or when impediments to the action of
the successive portions of the arteries exist, that
the blood is seen to move in their minute branches
in a pulsatory manner.
4. Want of Proof o f Irritability in the True
Capillaries.
It may now be asked, what presumption is there
of muscular contractile power, or irritability, in the
true capillaries ?
The flow of blood through the capillaries appears,
in every instance, to be effected and modified by
powers impressed upon it, of a character extraneous
to any action of these vessels themselves.
The influence of the contraction of the heart is