272 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



21. The hepatic portal system remains unchanged throughout. 



22. The ventral aorta persists as a single vessel in fishes and Amphibia. In reptile; 

 splits into three trunks, right and left aortae and pulmonary; in birds and mammals into t 

 trunks, the aorta and the pulmonary. 



23. The aortic arches are more or less modified in land vertebrates. In all of these 

 first two have vanished; and in all but certain Amphibia the fifth has likewise disappear 

 In these Amphibia (urodeles and Gymnophiona) the three (or four) remaining arches < 

 form complete arches, connecting with the dorsal aorta. In Anura, reptiles, birds, ; 

 mammals the third and sixth arches lose their connection with the dorsal aorta, only the fou 

 arches retaining this connection. The third arches persist as the bases of the carotid arter 

 the fourth arches form the aorta, and the bases of the sixth arches become the right and i 

 pulmonary arteries. In Anura and reptiles both fourth arches persist, forming right < 

 left aortic arches which unite dorsally to produce the dorsal aorta; but in birds the left fou 

 arch and in mammals the right fourth arch disappear. The right fourth in birds and the 1 

 fourth in mammals then remain as a single arch connecting the dorsal aorta with the ] 

 ventricle. The dorsal aorta in all vertebrates is the main artery of the body posterior to 

 heart. 



24. The pulmonary veins appear as new structures in the air-breathing vertebrates. Tl 

 enter the left auricle. Simultaneously there occurs a change in the heart and the double cir 

 lation is initiated. 



25. In Amphibia the four chambers of the heart are retained as in fishes, but the auri 

 is partially or completely- divided into right and left auricles. The sinus venosus is tl 

 attached to the right auricle, while the pulmonary veins enter the left auricle. The right s 

 of the heart consequently contains venous blood and the left side arterial blood. There is tl 

 a double circulation through the heart, but the two kinds of blood are imperfectly separat 



26. Above Amphibia the conus arteriosus is reduced to valves (the semilunar valves) 

 the bases of the great arteries which thereupon spring directly from the ventricles. 



27. In reptiles the sinus venosus is retained, the two auricles are completely separati 

 the ventricle is usually incompletely separated into right and left chambers. The dou 

 circulation is present but imperfect as in Amphibia. 



28. In birds and mammals the sinus venosus is reduced to a mere node in the wall of I 

 right auricle, the systemic veins then entering the right auricle directly. The ventricle 

 completely divided by a partition into right and left ventricles; consequently the two circu 

 tions through the heart are wholly separated. The right side of the heart is venous, the L 

 arterial. 



