general, this membrane is endued with a few
scattered capillary vessels only.
These various features of the minute and capillary
vessels, in the tail of the Gasterosteus acu-
leatus, or stickle-hack, are admirably pourtrayed
in Plate I. I have briefly described them as
they are seen in this species, in the first place,
because it presents the simplest example, and
consequently the type, of this interesting part of
the circulation. The structure and circulation in
the fin, are very similar to those of the tail. 2
2. Of the minute and capillary vessels in the
Web of the Frog ;— the Fana temporaria of
L in n æ u s .
The structure of the foot of the frog is far
more complicated than that of the fin or tail of
the fish. Designed for leaping, and in some
measure for apprehension, as well as for swimming,
the toes are of the most complicated
structure, and abundantly supplied with nervous,
muscular, tendinous, ligamentous, and osseous
textures. It is obvious that the minute and
capillary vessels must be distributed appropriately.'
The circulation, like the part, is of a
complicated character, and will require a more
particular description and delineation than that
of the fin or tail of the fish.
The larger arteries and veins of the frog’s foot,
run along the sides of the toes ; the former give
off, and the latter receive, about an equal number
of branches and roots to and from the web.
The minute arteries are characterized and distinguished
from the veins, by pursuing a straighter
course, generally across the web, by being smaller
in size, and lighter in colour, and by possessing
a more rapid circulation. On these accounts
they are apt to elude a cursory observation, and
have been described as only half as numerous
as the veins. The minute arteries and veins occur,
in reality, as seen by a high magnifying power*
in nearly equal numbers.
The minute veins are those vessels which, from
being more visible, first strike the eye of the
observer, on inspecting the circulation in the web
of the frog placed under the microscope. They
are tortuous, large, and red, and present the
most distinct view of the flow of the blood.
The motion of the blood in the arteries, in
the unimpeded circulation, is too rapid to admit