, where there are three branchial arches, the
bulb of the aorta splits into two trunks; each of these divides,
at first, into two branches, and then the posterior branch, on
each side, again subdivides into two others. Thus, three pairs
of aortic trunks are formed, which ascend upon the branchial
arches. The two anterior pairs of aortic trunks pass directly
into the roots of the dorsal aorta, but each gives off a vessel
which enters one of the external gills, the blood from which is
brought by an efferent canal into a higher part of the same
aortic arch. The third aortic trunk, on each side, is interrupted,
its lower part becoming the branchial artery of a gilltuft.
The blood is carried out of this branchia by a venous
trunk, which opens into the root of the dorsal aorta, and is, in
reality, merely the upper part of the third aortic trunk. The
facts may be expressed in another way, by saying that the
bases of the branchial artery and vein anastomose in the first
two gills, but not in the third.
The adult Axolotl (Siredon
) has four pairs of aortic trunks
(Fig. 25, E, p. 83); the hindermost pair (VI.) gives off the
pulmonary arteries, the three next (V., IV., III.) supply the external
branchiae; and the anterior trunk passes, above, into an
artery which divides into hyoidean and carotid branches.