I I . Of t h e m in u t e a n d c a p il l a r y v e s s e l s
OF THE PULMONARY CIRCULATION.
The character of the pulmonary circulation
is, like its object, remarkably different from that
of any part of the systemic circulation. As this
object is the change induced upon the blood
by its exposure to the influence of the atmospheric
air, so the manifest design in the pulmonary
structure, vessels, and circulation, is the
diffusion of the blood over the greatest possible
extent of surface.
1. Of the Circulation in the Lung of the
Salamander; — the Salamandre Crdte'e of C u v ie r .
The lung of the salamander is simply vesicular
; the distribution of its minute vessels is
equally distinguished by its simplicity, and by its
beautiful symmetry and parallelism.
A large artery on one side of the lung, is
exactly opposed to a large vein on the other.
The artery gives origin to branches which are
placed in an order perfectly parallel and symmetrical
with the corresponding minute veins. These
arterial branches sometimes divide into branches
still smaller. When this is the case, it is always
observed that venous roots, similar in form,
still, arise in parallel lines ; whilst, on the other
hand, when no such subdivision of the minute
arterial branches takes place, the corresponding
minute veins are equally destitute of these minuter
roots, and run in a manner simply lineal and
parallel with the artery.
We clearly discern the design of this arrangement.
The final arterial branches and the venous
origins are so placed, that the latter may most
commodiously receive the capillary vessels into
which the former ultimately issue, both being
placed in parallel lines.
This may be said to be the principal mode
of arrangement of the arteries and veins in the
lung of the salamander : the large and minute
branches and roots are placed in a state of
parallelism. There is a second mode of arrangement,
not less interesting to observe; some of
the last branches of the artery, instead of being
parallel with venous roots, pass into an angle or
space, formed by the union of two such roots or by
a flexure in the course of the minute vein itself.