284 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
undivided, the pulmonary arteries are connected with the same trunk 
(ventral aorta) as are the other aortic arches (fig. 280,C, D). In the 
amniotes (E, F, G, H) with partial or complete division of the ventricle, 
the truncus and the ventral aorta are divided in such a manner that 
derivatives of the sixth arch are connected with the right side of the heart, 
while the rest of the ventral aorta, save for the exception noted in the 
reptiles above, receives its blood from the left side of the heart. 
In connexion with the almost complete obliteration of the fifth arch, and in 
most pulmonate vertebrates, the separation of the sixth from the rest, it is interesting 
to note that in the lower vertebrates (elasmobranchs) there is already a differentia- 
tion of these two arches from the rest of the series (fig. 281). 
ARTERIES. 
The dorsal aorta arises from the fusion of two primitive trunks 
running approximately parallel to the notochord, and extends as a me- 
dian vessel, usually lying just dorsal to the origin of the mesentery, 
from the point of union of the radices back nearly to the posterior end 
of the body. 
In human anatomy the different parts of the aortic vessels have names different 
from those adopted here. The persistent portion of the ventral aorta is called the 
ascending aorta, the persistent fourth arch is the arch of the aorta, and the 
adjacent part of the dorsal aorta is the descending aorta. The rest of the dorsal 
aorta is divided into the thoracic and abdominal aorta, accordingly as they lie 
in the regions of the corresponding cavities. These terms are inapplicable in 
comparative anatomy. 
The arteries arising from the dorsal aorta may be grouped under the 
‘two categories, visceral and somatic (p. 268). To the former belong 
the vessels running through the mesenterial-like structures (mesen- 
teries, omenta, mesorchium, etc.) to supply the digestive tract and the 
excretory and reproductive organs. In the primitive condition those 
going to the alimentary canal are numerous but they do not show a meta- 
meric character. In the majority of vertebrates they become united 
into a smaller number of main trunks from which branches go to the 
various regions. ‘The principal of these trunks are the following: 
There is usually present a ceeliac artery, arising from the radix or 
from the dorsal aorta near it, and dividing in the mesogaster into 
gastric, splenic and hepatic arteries, distributed to stomach, spleen 
and liver. The superior mesenteric artery is connected in develop- 
