tive membranes, resemble, in their mode of distribution,
those of the fin and tail of the fish ; they
finally unite and form a minute venous root, which
eventually joins a large vein.
In the mesentery, I have once distinctly observed
the anastomosis of arteries. The contractions and
peristaltic motion of the intestines probably interrupt
the flow of the blood even through the arterial
branches, and render such anastomoses necessary
here, as would be superfluous in the web of the
frog.
I have also seen the artery give off a minute
branch, and this early to turn round and pursue a
venous course. I have not determined whether
such a vessel is a contorted artery, destined to
divide and form capillaries and to nourish the root
of the mesentery f or whether it really assumes the
character of a vein, joining other veins.
The distribution of the vessels of the mesentery
is given in Plate IY. The course of the large arteries
and veins, and of the capillary vessels, is
accurately marked. The former have an obvious
destination and origin beyond the mesentery respectively
; the latter are as obviously destined to
carry nutriment to the membrane itself.
To this representation of the course of the
minute and capillary vessels, in the mysentery,
are added two sketches of the artery and vein
of a frog; figures 2, and 3. Their object is to
point out the singular distribution of maculae
or spots along these vessels.
These maculae are similar to those of the skin,
and if carefully traced, may lead to some conclusion
as to the continuous character of the external
integuments, and the vascular texture. Into this
subject I have not had leisure to enter. It would,
I doubt not, amply repay the trouble of the investigation,
by some interesting results.
These maculae also enable us to trace the course
of the minute arteries and veins, and especially
the former, when it would otherwise, from their
empty state, be impossible. In this manner I have
been able to ascertain a singular, but interesting
anatomical fact, in regard to the relative situation
and course of the minute vessels and nerves,
which, after further research, I purpose to lay
before the profession.
D <z