If we institute a general comparison between
the systemic and pulmonary circulation, we
shall arrive at the following conclusions. The
arteries in the former divide and subdivide at
considerable intervals, until they become extremely
minute ; and from the rapidity of the
circulation, are only distinctly seen by the aid
of the higher powers of the microscope; in the
latter, the subdivisions of the minute arteries
take place at the nearest points along its course,
the arteries terminate abruptly, the branches
assume at once the capillary characters. The veins
are formed in a manner perfectly similar to that
of the division of the arteries, in the systemic
and pulmonary circulation respectively: the capillary
vessels of the systemic circulation are far
less numerous and more tortuous than those of
the lung. It may be said, that, in the web, the
vessels are adapted to support the nutrition and
life of its various textures; in the lung, that the
membrane is a mere scatfolding to spread the
vessels which convey the blood in the fullest
manner over its extensive surface.
2. Of the Circulation in the Lung of the Frog ;
the Rana temporaria of L i n n æ u s .
The lung of the frog is both vesicular and
cellular. It is this last structure which renders the
distribution of its vessels more complex and difficult
to follow, than those of the lung of the salamander.
The cells are formed by portions of
membrane situated vertically and internally. The
insertions of these membranes divide the external
surface of the lung into meshes ; the membranes
themselves are constituted into vertical meshes by
these mutual insertions.
Like the lung of the salamander, the lung of the
frog is endowed with a large artery and vein. The
course of these vessels is very peculiar. The former
passes along the more convex side of the lung,
along its surface ; the latter, along the less convex
side, and along the internal borders of the internal
meshes. The larger branches of the artery pass to
the right and left, and also occupy the surface
of the lung ; the larger veins pursue a similar
course along the internal borders of the internal
meshes.
This intricate relative disposition of external