must then become stagnant, an event which is
effectually prevented by the institution of a cross
current from the anastomosing venous roots, and
a retrograde current along one of the venous
trunks: and thus the death of the part is
prevented.
I have also sought in vain for any instance
of the immediate termination of an artery in a vein.
Generally, if not invariably, capillary vessels are
interposed. Such terminations have been said to
take place indeed ; but I am persuaded that the
appearance was delusive, and that, in reality, the
artery crossed, unseen, instead of entering into the
veins. This mistake is readily made : in the wTeb
of the frog, there are two distinct layers of vessels,
one above the other, and it requires the most
perfect microscope to detect the error which I have
noticed, and such an instrument was probably not
in the hands of the author of this observation.
The real mode of distribution of the minute
arteries is extremely interesting ; the precise mode
of their transition into capillaries has not been
described by any author.
The larger arteries first divide into branches.
These subdivide into still smaller branches, which
are also successively smaller than the trunk from
which they proceed. At length the singular fact
is observed, of each of the two branches being as
large, or even larger, than the vessel from which
they originate. At this point there is an obvious
and remarkable change in the appearance of the
circulation: the course of the blood becomes of
only half its former velocity, and the globules,
consequently, instead of moving too rapidly to be
seen, become distinctly visible. If the vessel be
traced, it is next observed, not to subdivide, but
to unite with other branches, and to pass into that
distinct system and net-work of vessels to which
I would restrict and appropriate the term capillary.
The object of this peculiar distinction and character
of the capillary vessels is very obvious : a more
diffused and slower circulation is required for
administering to the nutrient vessels or functions,
than that of the arteries; this peculiar character
of the circulation is conferred at once, by the subdivision
of the minute artery into branches of
equal size with itself.
Such is the phenomenon of the transition of the
arterial into the capillary vessels and circulation.
1 have examined it repeatedly with every power