FCETAL CIRCULATION. 



553 



Fig. 164.* 



directly backwards and joins the 

 inferior cava. In the inferior 

 cava the pure blood becomes 

 mixed with that which is return- 

 ing from the lower extremities, 

 and is carried through the right 

 auricle, guided by the Eustachian 

 valve, and through the foramen 

 ovate into the left auricle. From 

 the left auricle it passes into the 

 left ventricle, and from the left 

 ventricle into the aorta, whence 

 it is distributed, by means of the 

 carotid and subclavian arteries, 

 principally to the head and upper 

 extremities. From the head and 

 upper extremities, the impure blood 

 is returned by the superior vena 

 cava to the right auricle. From 

 the right auricle, it is propelled, 

 into the right ventricle; and 

 from the right ventricle into 

 the pulmonary artery. In the 

 adult, the blood would now be 

 circulated through the lungs, and 

 oxygenated ; but in the foetus the 

 lungs are solid, and almost imper- 

 vious. Only a small quantity of 

 the blood passes therefore into the 

 lungs; the greater part rushes 

 through the ductus arteriosus, into 

 the commencement of the descend- 

 ing aorta. 



* The foetal circulation. 1. The umbilical cord, consisting of the umbilical vein 

 and two umbilical arteries ; proceeding from the placenta (2). 3. The umbilical vein 

 dividing into three branches ; two (4, 4) to be distributed to the liver ; and one (5), the 

 ductus venosus, which enters the inferior vena cava (6). 7. The portal vein return- 

 ing the blood from the intestines, and uniting with the right hepatic branch. 8. 

 The right auricle ; the course of the blood is denoted by the arrow, proceeding from 8 

 to 9, the left auricle. 10. The left ventricle ; the blood following the arrow to the arch 

 of the aorta (11), to be distributed through the branches given off by the arch to the 

 head and upper extremities. The arrows 12 and 13, represent the return of the blood 

 from the head and upper extremities through the jugular and subclavian veins, to the 

 superior vena cava (14), to the right auricle (8), and in the course of the arrow through 

 the right ventricle (15), to the pulmonary artery (16). 17. The ductus arteriosus, 

 which appears to be a proper continuation of the pulmonary artery, the offsets at each 

 side are the right and left pulmonary artery cut off; these are of extremely small size 

 as compared with the ductus arteriosus. The ductus arteriosus joins the descending 

 aorta (18, 18), which divides into the common iliacs, and these into the internal iliacs, 

 which become the umbilical arteries (19), and return the blood along the umbilical 

 cord to the placenta ; while the other divisions, the external iliacs (20), are continued 

 into the lower extremities. The arrows at the termination of these vessels mark the 

 return of the venous blood by the veins to the inferior cava. 



70 







