FORMATION OF PERMANEXT "VESSELS. 795 



Vienna Acad., 1857), and although some 'points are still left in 

 doubt, their history may now be given from these observations, and the 

 supplemental illustration derived from the investigation of the various 

 examples of congenital malformation, the greater number of which are 

 manifestly related to variations in the natural mode of transformation. 

 This will be explained by reference to the diagram in fig. 593. 



From these researches it appears that the permanent vessels owe 

 their formation to the persiscence of certain of tlie foetal arches or 

 parts of them, while other arches or portions of them become oblite- 

 rated and disappear. Thus it is ascertained that in mammals the main 

 aortic arch, which in the adult passes to the left of the trachea and 

 gullet, is formed by the persistence of the fourth embryonic arterial arch 

 of the left side, which not only remains patent, and becomes con- 

 nected with tlie aortic stem of the arterial bulb, but keeps pace by its 

 increased width and the development of its walls with the rate of growth 

 in the other parts of the body, so that it soon surpasses all the rest 

 of the arches in its width of calibre and thickness of its walls. In 

 birds, however, the permanent aortic arch passes to the right of the 

 trachea and gullet, and it is formed by the persistence of the fourth 

 embryonic arch of the right side ; while, in all reptiles, as there are 

 two permanent aortic arches, it is by the persistence of both the right 

 and left foetal arches that the two aortas are produced, the right 

 being that which is most directly connected with the systemic or left 

 ventricle. 



The pulmonary arteries of mammals would appear by Rathke's 

 observations to be developed in connection with only one foetal arterial 

 arch, viz., the fifth of the left side, from the middle part of which 

 they appear as branches, and the whole fifth arch of the right side 

 undergoes rapid atrophy and ultimate obliteration. The first part of 

 the left fifth arch, becoming the common pulmonary artery, is connected 

 with that division of the arterial bulb which is separated as the pulmo- 

 nary stem ; but the remote part of this arch also remains fully patent, 

 and undergoing equally with the rest of it full development, continues 

 to lead into the left root of the aorta as ductus arteriosus Botalli, 

 which serves to convey the blood from the right ventricle of the foetal 

 heart into the descending aorta, but becomes obliterated at the time of 

 birth. 



This duct is therefore in mammals due to a persistent condition of 

 the fifth left branchial arch. But, in birds and reptiles, it appears that the 

 process of transformation is somewhat different, for in them the right 

 and left pulmonary arteries (excepting in those serpents in which there 

 is .only one lung developed) are formed in connection with the respec- 

 tive right and left fifth branchial arches, and there are thus two ductus 

 arteriosi during foetal life, the short one of the right side corresponding 

 to that which is left in mammals, and the longer one of the left side 

 passing round the pharynx into the left aortic root. Both of these 

 arches are obliterated at the time of the exclusion of the bird from 

 the eg^ ; but in some reptiles the ductus arteriosi remain permanently 

 open during life. 



The subclavian and vertebral arteries were shown by Rathke to spring 

 from the posterior aortic roots at a place between the junction of the 

 fourth and fifth arches. In mammals, the vessels on the left side are 

 from the first in direct connection Avith the aortic root at the place 



