Sometimes an arterial branch turns and crosses
the main trunk, in order to place itself parallel
with a venous root. Sometimes the minute branch
of the artery divides itself into two extreme branches
at a very obtuse angle, the lines of which are
opposed to similar lines formed by venous roots
equally uniting at an obtuse angle.
The whole of these arrangements are strictly
represented in Plate Y. The more the drawing is
examined, the more remarkable and numerous will
appear the instances of design displayed in the
arrangement of the pulmonary vessels.
The object of this peculiar disposition of the
minute arteries and veins of the lung is plainly
that the capillary vessels into which the final
branches of the former divide, may be received by
the similar and similarly-disposed roots of the
latter. The division of the minute arteries
into capillaries, takes place at once, without
any of those subdivisions observed in the arteries
of the systemic circulation seen in the web of
the frog. The arteries terminate, the capillaries
begin and terminate, and the veins commence,
equally abruptly. Between the final termination
of the artery, and the commencement of the vein,
there is a considerable space occupied by capillaries
infinitely more numerous than those of the web of
the frog, which diffuse the globules of blood over
the intervening space, affording a sight of the
most splendid description.
The final arteries give out, and the venous roots
receive the capillaries not only at their points, but
along their sides. This appears not to be the case
with the larger vessels. Along the course of these
in the dried lung there is a distinct space free from
globules, plainly denoting that the blood is not
directly received by them, but passes along or beyond
them to ulterior minute venous roots; their
course along or across the larger vessel is, indeed,
sometimes quite obvious in the living animal,
being marked by lines resembling the capillaries
in form.
The minute and capillary vessels and circulation
in the lung of the salamander, are the most distinct,
from its purely vesicular structure, and the want
of cells and vertical meshes which exist in the lung
of the other batrachia. It may therefore be taken
as affording the most distinct and perfect, because
the most simple type of the pulmonary circulation.
There is plainly no disposition to anastomosis