836 GENERATION. 



with the embryon being the omphalo-mesenteric arteries and veins. As 

 these processes are going on, the great, central vessel of the embryon becomes 

 enlarged and twisted upon itself, at a point just below the cephalic enlarge- 

 ment of the embryon, between the inferior extremity of the pharynx and the 

 superior cul-de-sac of the intestinal canal. The excavation which receives 

 this vessel is called the fovea cardiaca. Simple, undulatory movements take 

 place in the heart of the chick at about the middle of the second day ; but 

 there is not at that time any regular circulation. At the end of the second 

 day or the beginning of the third, the currents of the circulation are estab- 

 lished. The time of the first appearance of the circulation in the human 

 embryon has not been accurately determined. 



In the arrangement of the vessels for the first circulation in the embryon, 

 the heart is situated exactly in the median line and gives off two arches which 

 curve to either side and unite into a single central trunk at the spinal column 

 below. These are the two aortae, and the single trunk formed by their 

 union becomes the abdominal aorta. The two aortic arches, only one of 

 which is permanent, are sometimes called the inferior vertebral arteries. These 

 vessels give off a number of branches, which pass into the area vasculosa. 

 Two of these branches, however, are larger than the others, pass to the um- 

 bilical vesicle and are called the omphalo-mesenteric arteries. In the em- 

 bryon of mammals, there are at first four omphalo-mesenteric veins, two 

 superior, which are the larger, and two inferior ; but as development advances, 

 the two inferior veins are closed, and there are then two omphalo-mesen- 

 teric arteries and two omphalo-mesenteric veins. At about the fortieth day 

 one artery and one vein disappear, leaving one omphalo-mesenteric artery 

 and one vein. Soon after, as the circulation becomes established in the 

 allantois, the vessels of the umbilical vesicle and the omphalo-mesenteric 

 vessels are obliterated, and the first circulation is superseded by the second. 



As the septum between the two ventricles makes its appearance, that 

 division of the right aortic arch which constitutes the vascular portion of one 

 of the branchial arches disappears and loses its connection with the abdom- 

 inal aorta ; a branch, however, persists during the whole of intrauterine life 

 and constitutes the ductus arteriosus, and another branch is permanent, 

 forming th'e pulmonary artery. 



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

 sels disappear and as the allantois is developed to form the chorion, two 

 vessels (the hypogastric arteries) 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 con- 

 nected 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 disap- 

 pears, and there is finally but one vein in the umbilical cord. It is in this 

 way the umbilical arteries carrying the blood to the tufts of the foetal pla- 

 centa, which is returned by the umbilical vein that the placental circulation 

 is established. 



