ARTERIAL SYSTEM. 



649 



the root of the left subclavian. The truncus arteriosus is more- 

 over only divided into two parts, one of which is continuous 

 with all the systemic arteries. Thus it comes about that in 

 Birds the right fourth arch (e) alone gives rise to the dorsal 

 aorta. 



In Mammals (fig. 366 D) the truncus arteriosus is only 

 divided into two, but the left fourth arch (>), instead of the right, 

 is that continuous with the dorsal aorta, and the right fourth 

 arch (/) is only continued into the right vertebral and right 

 subclavian arteries. 



The fifth arch always gives origin to the pulmonary artery 

 (fig. 365, /) and is continuous with one of the divisions of the 

 truncus arteriosus. In Lizards (fig. 366 A, i), Chelonians and 

 Birds (fig. 366 C, i] and probably in Crocodilia, the right and 

 left pulmonary arteries spring respectively from the right and 

 left fifth arches, and during the greater part of embryonic life 

 the parts of the fifth arches between the origins of the pulmonary 

 arteries and the system of the dorsal aorta are preserved as 

 ductus Botalli. These ductus Botalli persist for life in the 

 Chelonia. In Ophidia (fig. 366 B, Ji) and Mammalia (fig. 

 366 D, m) only one of the fifth arches gives origin to the two 

 pulmonary arteries, viz. that on the right side in Ophidia, and 

 the left in Mammalia. 



The ductus Botalli of the fifth arch (known in Man as the 

 ductus arteriosus) of the side on which the pulmonary arteries 

 are formed, may remain (e.g. in Man) as a solid cord connecting 

 the common stem of the pulmonary aorta with the systemic 

 aorta. 



The main history of the arterial arches in the Amniota has 

 been sufficiently dealt with, and the diagram, fig. 366, copied 

 from Rathke, shews at a glance the character of the metamor- 

 phosis these arches undergo in the different types. It merely 

 remains for me to say a few words about the subclavian and 

 vertebral arteries. 



The subclavian arteries in Fishes usually spring from the 

 trunks connecting the branchial veins with the dorsal aorta. 

 This origin, which is also found in Amphibia, is typically found 

 in the embryos of the Amniota. In the Lizards this origin 

 persists through life, but both subclavians spring from the right 



