THE AORTIC SYSTEM. 101 



internal carotid. The dorsal ends of the third and fourth arches are still con- 

 nected, but this connection, instead of being a large aortic vessel, as in earlier 

 stages, has now contracted and almost disappeared, and will soon be lost altogether, 

 so that in the adult there will be no connection between the dorsal ends of the 

 third and fourth arches. The fifth arch is still connected with the dorsal end of 

 the fourth. It gives off the small pulmonary artery to the lungs. On the side 

 toward the heart the relations of the arches are also changed. The main aortic 

 vessel which springs from the heart is, in the 12 mm. pig, divided into two vessels 

 the pulmonary aorta on the ventral side and the true aorta in a more dorsal 

 position. The division has 'so taken place that the third and fourth arches are 

 connected only with the true aorta, while the fifth arch is connected only with the 

 pulmonary aorta. The part of the fifth arch on the left side between the origin 

 of the pulmonary artery proper and the main descending aorta offers at this stage 

 an open communication between the pathways of the pulmonary and of the main 

 body circulation. This dorsal half of the fifth aortic arch is known as the ductus 

 arteriosus. It remains throughout the fetal period as an open channel, so that the 

 blood from the right ventricle flows in part to the lungs, in part into the dorsal 

 aorta. The lumen of the ductus arteriosus disappears in man soon after birth. 

 As an anomaly it occasionally persists throughout life, involving serious modifica- 

 tions of the normal circulation. The dorsal part of the fifth aortic arch of the 

 right side has a different history, for it aborts early in embryonic life, and there 

 also occurs an abortion of the entire descending aorta from the end of the fourth 

 arch on the right side downward to the level of the diaphragm. When this abor- 

 tion has taken place, the entire aortic stream flows from the heart to the left side 

 of the embryo. The aortic branches on the right side appear as follows in the 

 adult: The main stem, from which the five arches originally sprang, is the arteria 

 innominata, which gives off a stem, the common carotid, from which spring the 

 two carotids of the right side. Next, a vessel which represents the persistent fourth 

 right arch, which no longer has any direct communication with the aorta, but at 

 its end gives off the subclavian and vertebral arteries. The vessel which corre- 

 sponds to the right fourth arch is usually described as a portion of the stem of the 

 subclavian artery in the adult. The aortic branches on the left side appeal, as 

 follows in the adult : first, the common carotid, the stem of the original first to 

 third arches; second, the subclavian, which includes a part of the fourth arch, but 

 consists chiefly of what was originally only a branch of the arch. The pulmonary 

 artery has as its stem a portion of the fifth arch, in man on the left side only, 

 but the vessel arises as a separate branch from the fifth arch. 



There often appear irregular vessels in early stages between the fourth and 

 fifth arches, and these have been held by some writers to represent an additional 

 partially aborted aortic arch. If this is the case, then the arch here called the 

 fifth would be more correctly termed the sixth. The arch supposed to be lost is 

 sometimes distinguished as Zimmermann 's arch. 



