off a single capillary vessel, to pursue its course for
a short distance, and then finally to divide into two
capillary vessels. The principal artery divides at
its extremity into four extreme branches, and each
of these into capillary vessels. The latter are
traced variously joining and forming venous roots.
The double layer of capillaries is made very
manifest by a simple device. Alcohol applied to
the web of the frog was found speedily to induce
a stagnant state of the blood in a certain portion
of the capillary vessels, whilst it moved in the arteries
and larger veins as before. I soon perceived
that it was in the capillaries of that layer which was
contiguous to the surface to which the alcohol was
applied, that the circulation was interrupted.
Figure 2, represents a minute portion of the web
seen in the microscope still more magnified, to the
upper surface of which alcohol had been applied.
The open lines are the upper layer of obstructed
capillaries ; the darker ones, the subjacent capillaries
carrying on the circulation freely.
Figure 3, represents a portion of the web to the
under side of which the alcohol had been applied.
The stagnant and circulation vessels occupy an
inverted position.
In order to view each series of capillaries
distinctly, it is necessary to adjust the microscope
so as to bring them severally into its focus.
The same device assists us also in distinguishing
the arteries and larger veins from the true
capillaries, the former remaining pervious after the
latter are obstructed.
3. Of the minute and capillary vessels in the
Mesentery of the Toad |— the Rana Bufo of
L in n a e u s .
The distribution of the vessels in the mesentery
is very peculiar. Nature’s object is not merely to
nourish this membrane, but to supply the intestines
with blood. The mesentery is, therefore,
chiefly the vehicle of an apparatus destined for an
ulterior object; a few nutrient vessels only are
distributed to the membrane itself.
On the first view of the circulation in the mesentery,
we observe large arteries and veins, marked
and distinguished by the directions of the fluid
flowing through them, pursuing their course across
the membrane. Occasionally a minute branch is
given off by an artery, which divides and becomes
capillary. The capillary vessels, like the respec-
D