at all, their occurrence is so rare, that, after the
most diligent search, I have not been able to
detect a single instance of them. And, from the
meagre and imperfect description given, and the
imperfect microscopes employed, by the authors
of these observations, I am persuaded that certain
delusive appearances, which I shall have occasion
to describe in the course of this paper, have been
mistaken for these supposed phenomena in the
distribution of the minute vessels.
The real course and distribution of the minute
arteries and veins, and of the capillary vessels, and
the real phenomena of the minute arterial, capillary,
and venous circulation, are, on the other
hand, so peculiar and interesting, that I think a
detailed and faithful account of them cannot prove
unacceptable to physiologists.
The point of greatest moment, in investigations
of this kind, is, however, perfect accuracy. I have
therefore taken the utmost pains to secure this
object. I have employed the achromatic microscope
of Mr. Dollond ; I have devised peculiar
modes of placing the moving scenes which I had
to examine, under its object-glass ; every fact has
been equally and repeatedly witnessed by Mr.
Henry Smith, who has assisted me most kindly
and ably, throughout the whole of this investigation
; and the anatomy has been accurately given
by the pencil of Mr. Hawkins.
It is essential, in the next place, in describing
the minute and capillary vessels and circulation,
that we should attach distinct ideas to the various
terms employed. It is, especially, quite necessary
to distinguish the capillary vessels from the minute
arteries from which they arise, and the minute
veins to which they give origin. From the want
of attention to the terms employed, much confusion
pervades the descriptions of those authors
who have treated of this subject.
The minute vessels may be considered as arterial,
as long as they continue to divide and subdivide
into smaller and smaller branches. The minute
veins are those vessels which gradually enlarge
from the successive addition of smaller roots.
The true capillary vessels are obviously distinct from
each of these. They do not become smaller by
subdivision, nor larger by conjunction; but they
are characterized by continual and successive union
and division, or anastomoses, whilst they retain
a nearly uniform diameter.
c