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JANE I. EOBERTSON. 
common stem of the fifth and sixth aortic arches. In the 
middle line the dorsal aortic roots meet and form the dorsal 
aorta (Text-fig. 2, Ao.). The fourth pair of aortic arches 
takes origin from the proximal part of the lateral ventral 
aorta on either side and curves outwards, upwards and inwards 
to its junction with the dorsal aortic root. The fifth and 
sixth pairs arise by short common stems from the proximal 
part of the ventral aorta. From these common stems the 
sixth aortic arches, which are short and of extremely small 
calibre, are given off almost immediately. The fifth and sixth 
aortic arches on either side curve round the postero-lateral 
regions of the mouth-cavity, and rejoin one another dorsally, 
entering the dorsal aortic roots near the middle line by a short 
common stem. The small size of the vessels of the sixth 
aortic arches is correlated with the fact that in the adult 
Lepidosiren the pulmonary arteries no longer arise directly 
from them as they did in the larva, but from the common stems 
of the fifth and sixth arches on either side, and apparently 
the fifth arch has taken on the larger share of the blood supply, 
while the sixth arch has dwindled to a very insignificant 
vessel. The channel by which arches 5 and 6 communicate 
with the dorsal aorta is relatively small compared with the 
lumen of the pulmonary artery, the main blood-stream passing 
into the latter vessel. In Lepidosiren all the aortic arches 
can be traced throughout as definite uninterrupted vessels. 
Yertebro-cerebral Arteries. — Immediately before the 
dorsal aortic roots join to form the dorsal aorta, two little 
vessels, the vertebro-cerebral arteries, pass from them one on 
either side to the base of the skull (Text-fig. 2, V.C.A.). 
Dorsal Aorta. — The dorsal aorta (Text-fig. 2, Ao.) is 
formed by the junction of the dorsal aortic roots far forwards 
about the level of the junction of the distal and transverse 
portions of the bulbus cordis. It extends throughout the 
length of the spinal column immediately ventral to it, and in 
the caudal region lies in the haemal canal, dorsal to the 
caudal vein. The main vessels arising from the dorsal 
aorta are the coeliac, subclavian, anterior and posterior 
