234 STRUCTURE OF THE VERTEBRATES 



noan fish. The first and second arches are lost, the base of the 

 third serving as the common carotid. The portion of each dorsal 

 aorta between the third and fourth arches is greatly reduced, 

 as is also the fifth arch. Large branches from the sixth arches 

 have grown posteriorly to enter the lungs as the pulmonary 

 arteries, and the dorsal segment of each sixth is greatly reduced. 

 Therefore, most of the blood entering the sixth branchials is 

 carried to the lungs, a small amount passing to the dorsal aorta 

 in an unaerated condition. The pharyngeal portion of the dorsal 

 aorta is paired, the two radices being formed by the fourth 

 branchial arches. 



The anuran amphibia are mor€ effectively specialized for 

 land life. On either side the connection between the third and 

 fourth arches has completely disappeared; the fifth arch is lost; 

 and there is no connection between the pulmonary artery and 

 the dorsal aorta. However, the pulmonary arteries remain as 

 branches of the single ventral aorta, causing a further mixture 

 of blood as it leaves the heart. The separation of aerated and 

 unaerated blood is due to the anterior position of the former in 

 the ventricle, and as the contraction of the heart forces it into 

 the ventral aorta the first blood takes the line of least resistance 

 into the large ventral vessel and the two radices of the aorta. 

 When the unaerated blood leaves the heart, pressure has de- 

 veloped in the aortae and the pulmonary vessels are filled. 



The typical reptile has a separation of the pulmonary arteries 

 from the aorta, the separation accompanying the development 

 of the ventricular septum. The pulmonary artery is cut from 

 the ventral aorta by a longitudinal constriction, so that the 

 two vessels leave the heart independently. Due to the structure 

 of the ventricular septum the pulmonary vessel carries mostly 

 unaerated blood. 



In most reptiles the right radix of the aorta is larger and 

 filled with almost unmixed blood. It is from this vessel that the 

 carotids arise. The left radix carries partially mixed blood which 

 is emptied into the dorsal aorta and carried to the tissues. The 

 alligator has the most complete separation of blood of any 

 reptile, the small opening between the pulmonary and aortic 

 vessels permitting of only slight mixture. 



In the birds the separation of aerated and unaerated blood is 



