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 aortse, 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 coeliac 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- 



