between the arteries, or to direct communication
between the arteries and veins. There is uniformly
an intermediate space ; and this space is
occupied by an intermediate set of vessels.
The final branches of the pulmonary arteries
exist as rather large trunks, and then divide at
once, both from their sides and extremities into
innumerable capillary vessels. These like all other
capillaries preserve one uniform size. They are
far more numerous in proportion to the minute
arteries and veins, and to the spaces over which
they are spread than those of the systemic circulation.
On a cursory glance with a lens of moderate
power, the pulmonary capillary circulation seems
to be one uniform rush or ■ flood of blood flowingO with extreme rapidity in straight lines directly
across the pulmonary membrane originating from
a minute artery, diverging as rays from a point or
line, as a centre, and converging in a similar manner
to constitute a vein of similar form. The
general idea conveyed is that of a splendid device
for spreading the moving globules of blood over
the greatest extent of surface possible.
It is only on employing a higher power that the
frequent or rather continual anastomoses, junctions,
and subdivisions of the capillary vessels become
perfectly and fully visible. With such a power
the minute arteries are seen to terminate abruptly
in a net-work of capillary vessels, the communications
of which are, as I have said, infinitely
more frequent than those of the capillaries of the
web. Along these vessels and their anastomoses,
the globules of the blood are observed moving in
every possible direction, spreading themselves over
every point of the pulmonary membrane from a
minute arterial branch or extremity, or converging
in a similar manner to the point or sides of a
minute vein, in a way quite peculiar to the pulmonary
circulation. The capillary vessels of the lung
appear to have a general course in a direct line
from the minute artery, from which they arise,
to the corresponding vein ; but in this course they
are really slightly tortuous, or waving, and
retiform. It is altogether a scene very different
from that of the involved and irregular tortuosities
of the capillaries of the general circulation and will
be best conceived on an inspection of Plate VIII.,
to which I shall have occasion to refer more
particularly shortly.