DEVELOPMENT OF THE CIRCULATORY SYSTEM. 



933 



The Second, or Placental Circulation. As the omphalo-mesenteric vessels disappear, 

 and as the allantois is developed to form the chorion, two vessels (the hypogastric arte- 

 ries) are given off, first from the abdominal aorta ; but afterward, as the vessels going to 

 the lower extremities are developed, the branching of the abdominal aorta is such that 

 the vessels become connected with the internal iliac arteries. The hypogastric arteries 

 pass to the chorion through the umbilical cord and constitute the two umbilical arteries. 

 At first, there are two umbilical veins; but one of them afterward disappears, and there 

 is finally but one vein in the umbilical cord. It is in this way the umbilical arteries car- 

 rying the blood to the tufts of the foetal placenta, which is returned by the umbilical 

 vein that the placental circulation is established. 



Corresponding to the four visceral arches, which we have described in connection with 

 the development of the face, are four vascular arches. One of these disappears, and the 

 remaining three undergo certain changes, by which they are converted into the vessels 

 going to the head and the superior extremities. The anterior arches on the two sides 

 are converted into the carotids and subclavians ; the second, on the left side, is converted 

 into the permanent aorta, and the right is obliterated ; the third, on either side, is con- 

 verted into the right and left pulmonary arteries. In the early stages of the develop- 

 ment of the vascular system of mammals, the conditions have been compared to the per- 

 manent arrangement of the circulatory system in fishes. The heart of fishes remains 

 single ; and the heart of mammals is at first single, but afterward it becomes divided by the 

 development of the intra-ventricular septum. The branchial arches in fishes are perma- 

 nent, they receive all the blood from the aortic bulb, and the blood 

 from these arches then passes into the dorsal aorta. This is very 

 nearly the condition of the vascular system when the branchial 

 arches first appear in the embryon of mammals. 



The changes of the branchial arches which we have described 

 are illustrated in the diagrammatic Fig. 310. In this figure, the 

 three branchial arches that remain and participate in the devel- 

 opment of the upper portion of the vascular system are 1, 2, 3, 

 on either side. The two anterior (3, 3) become the carotids (c, c) 

 and the subclavians (s, ). The second (2, 2) is obliterated on the 

 right side, and becomes the arch of the aorta on the left side. 

 The third (1, 1), counting from above downward, is converted 

 into the pulmonary arteries of the two sides. Upon the left side, 

 there is a large anastomosing vessel (ca), between the pulmonary 

 artery of that side and the arch of the aorta, which is the ductus 

 arteriosus. The anastomosing vessel (cd) between the right pul- 

 monary artery and the aorta, is obliterated. 



The mode of development of the veins is very simple. Two 

 venous trunks make their appearance by the sides of the spinal 

 column, which are called the cardinal veins, and run parallel with 

 the superior vertebral arteries, or the two aortae, emptying finally 

 into the auricular portion of the heart by two canals, which are 

 called the canals of Cuvier. These veins change their relations 

 and connections as the first circulation is replaced by the second. 

 The omphalo-mesenteric vein opens into the heart between the 

 two canals of Cuvier. As development advances, the liver is 

 formed in the course of this vessel, a short distance below the 

 heart, and the vein ramifies in its substance ; so that the blood of 

 the omphalo-mesenteric vein passes through the liver before it 

 gets to the heart. We have seen that the omphalo-mesenteric vein is obliterated as the 

 umbilical vein makes its appearance. The blood from the umbilical vein is at first 

 emptied directly into the heart ; but this vessel soon establishes the same relations 



FIG. 810. Transformation 

 of the system of aortic 

 arches into permanent 

 arterial trunks, in the 

 mammalia. (Von Baer.) 



B, aortic bulb : 1, 2. 3, 4, 5, on 

 either side, the five pairs of 

 aortic arches ; 5, 5, the earli- 

 est in their appearance; 1. 1, 

 the most recent; c, c, the 

 two carotids, still united, 

 which are separated at a 

 later period; , , the two 

 subclavians, the ri^rlit aris- 

 ing' from the arteria innonii- 

 nata ; a. </. the aorta; p, p, 

 the pulmonary arteries; ca, 

 the ductus arteirosus ; c<7, 

 the left artieral canal, which 

 is finally obliterated. 



