168 STUDY OF PIG EMBRYOS. 



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 foetal 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 modifi- 

 cations 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 

 abortion 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 from which spring the two carotids. 

 Next a vessel which represents the persistent third 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 corresponds to the right third arch is 

 usually described as a portion of the stem of the subclavian artery in the adult. 



Figure 105 is in many respects similar to figure 101 , and is intended to show 

 chiefly the disposition of the veins. There are also included in this fig'ure the 

 Wolffian body and its duct. The pharynx and the heart are supposed to have 

 been cut through, well to the right of the median plane. This makes it possible 

 to indicate in the figure the origin of the pulmonary aorta and of the true aorta. 

 The following are the most important veins: the umbilical, which passes around 

 the umbilical opening and enters the liver; the portal vein, which receives the 

 blood from the abdominal viscera, and also delivers it to the liver. In this speci- 

 men there is quite a wide and free connection within the liver between the portal 

 and umbilical veins. In other embryos of this size such a connection does not 

 always exist. The large vena cava inferior is on the right side of the embryo, 

 and also shows the liver, which receives blood directly from the Wolffian bodies 

 and from the cardinal veins. From the upper side of the liver the hepatic vein 

 goes directly to the heart, uniting with the ducts of Cuvier. These ducts receive 

 the jugular veins from the head and the cardinal veins from the body. The car- 

 dinal veins are now very much changed. In earlier stages they extended from 

 the duct of Cuvier almost the entire length of the embryo. Of this great vessel 

 there now remains connected with the duct of Cuvier only a comparatively short 

 vessel. 



Figure 107 shows the disposition of the cephalic and upper cervical nerves 

 and also the position of the nasal cavity, the eye, and the otocyst. 



