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 reptiles it 

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

 trunks, the aorta and the pulmonary. 



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

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

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

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

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

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

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

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

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

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

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

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

 heart. 



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

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

 lation is initiated. 



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

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

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

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

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



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

 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 separated, 

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

 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 the 

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

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

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

 arterial. 



